                          Lua Lua 5.4 Reference Manual

   by Roberto Ierusalimschy, Luiz Henrique de Figueiredo, Waldemar Celes

   Copyright © 2020–2022 Lua.org, PUC-Rio. Freely available under the terms
   of the Lua license.
   contents · index · other versions

                                1 – Introduction

   Lua is a powerful, efficient, lightweight, embeddable scripting language.
   It supports procedural programming, object-oriented programming,
   functional programming, data-driven programming, and data description.

   Lua combines simple procedural syntax with powerful data description
   constructs based on associative arrays and extensible semantics. Lua is
   dynamically typed, runs by interpreting bytecode with a register-based
   virtual machine, and has automatic memory management with a generational
   garbage collection, making it ideal for configuration, scripting, and
   rapid prototyping.

   Lua is implemented as a library, written in clean C, the common subset of
   Standard C and C++. The Lua distribution includes a host program called
   lua, which uses the Lua library to offer a complete, standalone Lua
   interpreter, for interactive or batch use. Lua is intended to be used both
   as a powerful, lightweight, embeddable scripting language for any program
   that needs one, and as a powerful but lightweight and efficient
   stand-alone language.

   As an extension language, Lua has no notion of a "main" program: it works
   embedded in a host client, called the embedding program or simply the
   host. (Frequently, this host is the stand-alone lua program.) The host
   program can invoke functions to execute a piece of Lua code, can write and
   read Lua variables, and can register C functions to be called by Lua code.
   Through the use of C functions, Lua can be augmented to cope with a wide
   range of different domains, thus creating customized programming languages
   sharing a syntactical framework.

   Lua is free software, and is provided as usual with no guarantees, as
   stated in its license. The implementation described in this manual is
   available at Lua's official web site, www.lua.org.

   Like any other reference manual, this document is dry in places. For a
   discussion of the decisions behind the design of Lua, see the technical
   papers available at Lua's web site. For a detailed introduction to
   programming in Lua, see Roberto's book, Programming in Lua.

                               2 – Basic Concepts

   This section describes the basic concepts of the language.

2.1 – Values and Types

   Lua is a dynamically typed language. This means that variables do not have
   types; only values do. There are no type definitions in the language. All
   values carry their own type.

   All values in Lua are first-class values. This means that all values can
   be stored in variables, passed as arguments to other functions, and
   returned as results.

   There are eight basic types in Lua: nil, boolean, number, string,
   function, userdata, thread, and table. The type nil has one single value,
   nil, whose main property is to be different from any other value; it often
   represents the absence of a useful value. The type boolean has two values,
   false and true. Both nil and false make a condition false; they are
   collectively called false values. Any other value makes a condition true.
   Despite its name, false is frequently used as an alternative to nil, with
   the key difference that false behaves like a regular value in a table,
   while a nil in a table represents an absent key.

   The type number represents both integer numbers and real (floating-point)
   numbers, using two subtypes: integer and float. Standard Lua uses 64-bit
   integers and double-precision (64-bit) floats, but you can also compile
   Lua so that it uses 32-bit integers and/or single-precision (32-bit)
   floats. The option with 32 bits for both integers and floats is
   particularly attractive for small machines and embedded systems. (See
   macro LUA_32BITS in file luaconf.h.)

   Unless stated otherwise, any overflow when manipulating integer values
   wrap around, according to the usual rules of two-complement arithmetic.
   (In other words, the actual result is the unique representable integer
   that is equal modulo 2^n to the mathematical result, where n is the number
   of bits of the integer type.)

   Lua has explicit rules about when each subtype is used, but it also
   converts between them automatically as needed (see §3.4.3). Therefore, the
   programmer may choose to mostly ignore the difference between integers and
   floats or to assume complete control over the representation of each
   number.

   The type string represents immutable sequences of bytes. Lua is 8-bit
   clean: strings can contain any 8-bit value, including embedded zeros
   ('\0'). Lua is also encoding-agnostic; it makes no assumptions about the
   contents of a string. The length of any string in Lua must fit in a Lua
   integer.

   Lua can call (and manipulate) functions written in Lua and functions
   written in C (see §3.4.10). Both are represented by the type function.

   The type userdata is provided to allow arbitrary C data to be stored in
   Lua variables. A userdata value represents a block of raw memory. There
   are two kinds of userdata: full userdata, which is an object with a block
   of memory managed by Lua, and light userdata, which is simply a C pointer
   value. Userdata has no predefined operations in Lua, except assignment and
   identity test. By using metatables, the programmer can define operations
   for full userdata values (see §2.4). Userdata values cannot be created or
   modified in Lua, only through the C API. This guarantees the integrity of
   data owned by the host program and C libraries.

   The type thread represents independent threads of execution and it is used
   to implement coroutines (see §2.6). Lua threads are not related to
   operating-system threads. Lua supports coroutines on all systems, even
   those that do not support threads natively.

   The type table implements associative arrays, that is, arrays that can
   have as indices not only numbers, but any Lua value except nil and NaN.
   (Not a Number is a special floating-point value used by the IEEE 754
   standard to represent undefined numerical results, such as 0/0.) Tables
   can be heterogeneous; that is, they can contain values of all types
   (except nil). Any key associated to the value nil is not considered part
   of the table. Conversely, any key that is not part of a table has an
   associated value nil.

   Tables are the sole data-structuring mechanism in Lua; they can be used to
   represent ordinary arrays, lists, symbol tables, sets, records, graphs,
   trees, etc. To represent records, Lua uses the field name as an index. The
   language supports this representation by providing a.name as syntactic
   sugar for a["name"]. There are several convenient ways to create tables in
   Lua (see §3.4.9).

   Like indices, the values of table fields can be of any type. In
   particular, because functions are first-class values, table fields can
   contain functions. Thus tables can also carry methods (see §3.4.11).

   The indexing of tables follows the definition of raw equality in the
   language. The expressions a[i] and a[j] denote the same table element if
   and only if i and j are raw equal (that is, equal without metamethods). In
   particular, floats with integral values are equal to their respective
   integers (e.g., 1.0 == 1). To avoid ambiguities, any float used as a key
   that is equal to an integer is converted to that integer. For instance, if
   you write a[2.0] = true, the actual key inserted into the table will be
   the integer 2.

   Tables, functions, threads, and (full) userdata values are objects:
   variables do not actually contain these values, only references to them.
   Assignment, parameter passing, and function returns always manipulate
   references to such values; these operations do not imply any kind of copy.

   The library function type returns a string describing the type of a given
   value (see type).

2.2 – Environments and the Global Environment

   As we will discuss further in §3.2 and §3.3.3, any reference to a free
   name (that is, a name not bound to any declaration) var is syntactically
   translated to _ENV.var. Moreover, every chunk is compiled in the scope of
   an external local variable named _ENV (see §3.3.2), so _ENV itself is
   never a free name in a chunk.

   Despite the existence of this external _ENV variable and the translation
   of free names, _ENV is a completely regular name. In particular, you can
   define new variables and parameters with that name. Each reference to a
   free name uses the _ENV that is visible at that point in the program,
   following the usual visibility rules of Lua (see §3.5).

   Any table used as the value of _ENV is called an environment.

   Lua keeps a distinguished environment called the global environment. This
   value is kept at a special index in the C registry (see §4.3). In Lua, the
   global variable _G is initialized with this same value. (_G is never used
   internally, so changing its value will affect only your own code.)

   When Lua loads a chunk, the default value for its _ENV variable is the
   global environment (see load). Therefore, by default, free names in Lua
   code refer to entries in the global environment and, therefore, they are
   also called global variables. Moreover, all standard libraries are loaded
   in the global environment and some functions there operate on that
   environment. You can use load (or loadfile) to load a chunk with a
   different environment. (In C, you have to load the chunk and then change
   the value of its first upvalue; see lua_setupvalue.)

2.3 – Error Handling

   Several operations in Lua can raise an error. An error interrupts the
   normal flow of the program, which can continue by catching the error.

   Lua code can explicitly raise an error by calling the error function.
   (This function never returns.)

   To catch errors in Lua, you can do a protected call, using pcall (or
   xpcall). The function pcall calls a given function in protected mode. Any
   error while running the function stops its execution, and control returns
   immediately to pcall, which returns a status code.

   Because Lua is an embedded extension language, Lua code starts running by
   a call from C code in the host program. (When you use Lua standalone, the
   lua application is the host program.) Usually, this call is protected; so,
   when an otherwise unprotected error occurs during the compilation or
   execution of a Lua chunk, control returns to the host, which can take
   appropriate measures, such as printing an error message.

   Whenever there is an error, an error object is propagated with information
   about the error. Lua itself only generates errors whose error object is a
   string, but programs may generate errors with any value as the error
   object. It is up to the Lua program or its host to handle such error
   objects. For historical reasons, an error object is often called an error
   message, even though it does not have to be a string.

   When you use xpcall (or lua_pcall, in C) you may give a message handler to
   be called in case of errors. This function is called with the original
   error object and returns a new error object. It is called before the error
   unwinds the stack, so that it can gather more information about the error,
   for instance by inspecting the stack and creating a stack traceback. This
   message handler is still protected by the protected call; so, an error
   inside the message handler will call the message handler again. If this
   loop goes on for too long, Lua breaks it and returns an appropriate
   message. The message handler is called only for regular runtime errors. It
   is not called for memory-allocation errors nor for errors while running
   finalizers or other message handlers.

   Lua also offers a system of warnings (see warn). Unlike errors, warnings
   do not interfere in any way with program execution. They typically only
   generate a message to the user, although this behavior can be adapted from
   C (see lua_setwarnf).

2.4 – Metatables and Metamethods

   Every value in Lua can have a metatable. This metatable is an ordinary Lua
   table that defines the behavior of the original value under certain
   events. You can change several aspects of the behavior of a value by
   setting specific fields in its metatable. For instance, when a non-numeric
   value is the operand of an addition, Lua checks for a function in the
   field __add of the value's metatable. If it finds one, Lua calls this
   function to perform the addition.

   The key for each event in a metatable is a string with the event name
   prefixed by two underscores; the corresponding value is called a
   metavalue. For most events, the metavalue must be a function, which is
   then called a metamethod. In the previous example, the key is the string
   "__add" and the metamethod is the function that performs the addition.
   Unless stated otherwise, a metamethod may in fact be any callable value,
   which is either a function or a value with a __call metamethod.

   You can query the metatable of any value using the getmetatable function.
   Lua queries metamethods in metatables using a raw access (see rawget).

   You can replace the metatable of tables using the setmetatable function.
   You cannot change the metatable of other types from Lua code, except by
   using the debug library (§6.10).

   Tables and full userdata have individual metatables, although multiple
   tables and userdata can share their metatables. Values of all other types
   share one single metatable per type; that is, there is one single
   metatable for all numbers, one for all strings, etc. By default, a value
   has no metatable, but the string library sets a metatable for the string
   type (see §6.4).

   A detailed list of operations controlled by metatables is given next. Each
   event is identified by its corresponding key. By convention, all metatable
   keys used by Lua are composed by two underscores followed by lowercase
   Latin letters.
     * __add: the addition (+) operation. If any operand for an addition is
       not a number, Lua will try to call a metamethod. It starts by checking
       the first operand (even if it is a number); if that operand does not
       define a metamethod for __add, then Lua will check the second operand.
       If Lua can find a metamethod, it calls the metamethod with the two
       operands as arguments, and the result of the call (adjusted to one
       value) is the result of the operation. Otherwise, if no metamethod is
       found, Lua raises an error.
     * __sub: the subtraction (-) operation. Behavior similar to the addition
       operation.
     * __mul: the multiplication (*) operation. Behavior similar to the
       addition operation.
     * __div: the division (/) operation. Behavior similar to the addition
       operation.
     * __mod: the modulo (%) operation. Behavior similar to the addition
       operation.
     * __pow: the exponentiation (^) operation. Behavior similar to the
       addition operation.
     * __unm: the negation (unary -) operation. Behavior similar to the
       addition operation.
     * __idiv: the floor division (//) operation. Behavior similar to the
       addition operation.
     * __band: the bitwise AND (&) operation. Behavior similar to the
       addition operation, except that Lua will try a metamethod if any
       operand is neither an integer nor a float coercible to an integer (see
       §3.4.3).
     * __bor: the bitwise OR (|) operation. Behavior similar to the bitwise
       AND operation.
     * __bxor: the bitwise exclusive OR (binary ~) operation. Behavior
       similar to the bitwise AND operation.
     * __bnot: the bitwise NOT (unary ~) operation. Behavior similar to the
       bitwise AND operation.
     * __shl: the bitwise left shift (<<) operation. Behavior similar to the
       bitwise AND operation.
     * __shr: the bitwise right shift (>>) operation. Behavior similar to the
       bitwise AND operation.
     * __concat: the concatenation (..) operation. Behavior similar to the
       addition operation, except that Lua will try a metamethod if any
       operand is neither a string nor a number (which is always coercible to
       a string).
     * __len: the length (#) operation. If the object is not a string, Lua
       will try its metamethod. If there is a metamethod, Lua calls it with
       the object as argument, and the result of the call (always adjusted to
       one value) is the result of the operation. If there is no metamethod
       but the object is a table, then Lua uses the table length operation
       (see §3.4.7). Otherwise, Lua raises an error.
     * __eq: the equal (==) operation. Behavior similar to the addition
       operation, except that Lua will try a metamethod only when the values
       being compared are either both tables or both full userdata and they
       are not primitively equal. The result of the call is always converted
       to a boolean.
     * __lt: the less than (<) operation. Behavior similar to the addition
       operation, except that Lua will try a metamethod only when the values
       being compared are neither both numbers nor both strings. Moreover,
       the result of the call is always converted to a boolean.
     * __le: the less equal (<=) operation. Behavior similar to the less than
       operation.
     * __index: The indexing access operation table[key]. This event happens
       when table is not a table or when key is not present in table. The
       metavalue is looked up in the metatable of table.

       The metavalue for this event can be either a function, a table, or any
       value with an __index metavalue. If it is a function, it is called
       with table and key as arguments, and the result of the call (adjusted
       to one value) is the result of the operation. Otherwise, the final
       result is the result of indexing this metavalue with key. This
       indexing is regular, not raw, and therefore can trigger another
       __index metavalue.
     * __newindex: The indexing assignment table[key] = value. Like the index
       event, this event happens when table is not a table or when key is not
       present in table. The metavalue is looked up in the metatable of
       table.

       Like with indexing, the metavalue for this event can be either a
       function, a table, or any value with an __newindex metavalue. If it is
       a function, it is called with table, key, and value as arguments.
       Otherwise, Lua repeats the indexing assignment over this metavalue
       with the same key and value. This assignment is regular, not raw, and
       therefore can trigger another __newindex metavalue.

       Whenever a __newindex metavalue is invoked, Lua does not perform the
       primitive assignment. If needed, the metamethod itself can call rawset
       to do the assignment.
     * __call: The call operation func(args). This event happens when Lua
       tries to call a non-function value (that is, func is not a function).
       The metamethod is looked up in func. If present, the metamethod is
       called with func as its first argument, followed by the arguments of
       the original call (args). All results of the call are the results of
       the operation. This is the only metamethod that allows multiple
       results.

   In addition to the previous list, the interpreter also respects the
   following keys in metatables: __gc (see §2.5.3), __close (see §3.3.8),
   __mode (see §2.5.4), and __name. (The entry __name, when it contains a
   string, may be used by tostring and in error messages.)

   For the unary operators (negation, length, and bitwise NOT), the
   metamethod is computed and called with a dummy second operand, equal to
   the first one. This extra operand is only to simplify Lua's internals (by
   making these operators behave like a binary operation) and may be removed
   in future versions. For most uses this extra operand is irrelevant.

   Because metatables are regular tables, they can contain arbitrary fields,
   not only the event names defined above. Some functions in the standard
   library (e.g., tostring) use other fields in metatables for their own
   purposes.

   It is a good practice to add all needed metamethods to a table before
   setting it as a metatable of some object. In particular, the __gc
   metamethod works only when this order is followed (see §2.5.3). It is also
   a good practice to set the metatable of an object right after its
   creation.

2.5 – Garbage Collection

   Lua performs automatic memory management. This means that you do not have
   to worry about allocating memory for new objects or freeing it when the
   objects are no longer needed. Lua manages memory automatically by running
   a garbage collector to collect all dead objects. All memory used by Lua is
   subject to automatic management: strings, tables, userdata, functions,
   threads, internal structures, etc.

   An object is considered dead as soon as the collector can be sure the
   object will not be accessed again in the normal execution of the program.
   ("Normal execution" here excludes finalizers, which can resurrect dead
   objects (see §2.5.3), and excludes also operations using the debug
   library.) Note that the time when the collector can be sure that an object
   is dead may not coincide with the programmer's expectations. The only
   guarantees are that Lua will not collect an object that may still be
   accessed in the normal execution of the program, and it will eventually
   collect an object that is inaccessible from Lua. (Here, inaccessible from
   Lua means that neither a variable nor another live object refer to the
   object.) Because Lua has no knowledge about C code, it never collects
   objects accessible through the registry (see §4.3), which includes the
   global environment (see §2.2).

   The garbage collector (GC) in Lua can work in two modes: incremental and
   generational.

   The default GC mode with the default parameters are adequate for most
   uses. However, programs that waste a large proportion of their time
   allocating and freeing memory can benefit from other settings. Keep in
   mind that the GC behavior is non-portable both across platforms and across
   different Lua releases; therefore, optimal settings are also non-portable.

   You can change the GC mode and parameters by calling lua_gc in C or
   collectgarbage in Lua. You can also use these functions to control the
   collector directly (e.g., to stop and restart it).

  2.5.1 – Incremental Garbage Collection

   In incremental mode, each GC cycle performs a mark-and-sweep collection in
   small steps interleaved with the program's execution. In this mode, the
   collector uses three numbers to control its garbage-collection cycles: the
   garbage-collector pause, the garbage-collector step multiplier, and the
   garbage-collector step size.

   The garbage-collector pause controls how long the collector waits before
   starting a new cycle. The collector starts a new cycle when the use of
   memory hits n% of the use after the previous collection. Larger values
   make the collector less aggressive. Values equal to or less than 100 mean
   the collector will not wait to start a new cycle. A value of 200 means
   that the collector waits for the total memory in use to double before
   starting a new cycle. The default value is 200; the maximum value is 1000.

   The garbage-collector step multiplier controls the speed of the collector
   relative to memory allocation, that is, how many elements it marks or
   sweeps for each kilobyte of memory allocated. Larger values make the
   collector more aggressive but also increase the size of each incremental
   step. You should not use values less than 100, because they make the
   collector too slow and can result in the collector never finishing a
   cycle. The default value is 100; the maximum value is 1000.

   The garbage-collector step size controls the size of each incremental
   step, specifically how many bytes the interpreter allocates before
   performing a step. This parameter is logarithmic: A value of n means the
   interpreter will allocate 2^n bytes between steps and perform equivalent
   work during the step. A large value (e.g., 60) makes the collector a
   stop-the-world (non-incremental) collector. The default value is 13, which
   means steps of approximately 8 Kbytes.

  2.5.2 – Generational Garbage Collection

   In generational mode, the collector does frequent minor collections, which
   traverses only objects recently created. If after a minor collection the
   use of memory is still above a limit, the collector does a stop-the-world
   major collection, which traverses all objects. The generational mode uses
   two parameters: the minor multiplier and the the major multiplier.

   The minor multiplier controls the frequency of minor collections. For a
   minor multiplier x, a new minor collection will be done when memory grows
   x% larger than the memory in use after the previous major collection. For
   instance, for a multiplier of 20, the collector will do a minor collection
   when the use of memory gets 20% larger than the use after the previous
   major collection. The default value is 20; the maximum value is 200.

   The major multiplier controls the frequency of major collections. For a
   major multiplier x, a new major collection will be done when memory grows
   x% larger than the memory in use after the previous major collection. For
   instance, for a multiplier of 100, the collector will do a major
   collection when the use of memory gets larger than twice the use after the
   previous collection. The default value is 100; the maximum value is 1000.

  2.5.3 – Garbage-Collection Metamethods

   You can set garbage-collector metamethods for tables and, using the C API,
   for full userdata (see §2.4). These metamethods, called finalizers, are
   called when the garbage collector detects that the corresponding table or
   userdata is dead. Finalizers allow you to coordinate Lua's garbage
   collection with external resource management such as closing files,
   network or database connections, or freeing your own memory.

   For an object (table or userdata) to be finalized when collected, you must
   mark it for finalization. You mark an object for finalization when you set
   its metatable and the metatable has a __gc metamethod. Note that if you
   set a metatable without a __gc field and later create that field in the
   metatable, the object will not be marked for finalization.

   When a marked object becomes dead, it is not collected immediately by the
   garbage collector. Instead, Lua puts it in a list. After the collection,
   Lua goes through that list. For each object in the list, it checks the
   object's __gc metamethod: If it is present, Lua calls it with the object
   as its single argument.

   At the end of each garbage-collection cycle, the finalizers are called in
   the reverse order that the objects were marked for finalization, among
   those collected in that cycle; that is, the first finalizer to be called
   is the one associated with the object marked last in the program. The
   execution of each finalizer may occur at any point during the execution of
   the regular code.

   Because the object being collected must still be used by the finalizer,
   that object (and other objects accessible only through it) must be
   resurrected by Lua. Usually, this resurrection is transient, and the
   object memory is freed in the next garbage-collection cycle. However, if
   the finalizer stores the object in some global place (e.g., a global
   variable), then the resurrection is permanent. Moreover, if the finalizer
   marks a finalizing object for finalization again, its finalizer will be
   called again in the next cycle where the object is dead. In any case, the
   object memory is freed only in a GC cycle where the object is dead and not
   marked for finalization.

   When you close a state (see lua_close), Lua calls the finalizers of all
   objects marked for finalization, following the reverse order that they
   were marked. If any finalizer marks objects for collection during that
   phase, these marks have no effect.

   Finalizers cannot yield nor run the garbage collector. Because they can
   run in unpredictable times, it is good practice to restrict each finalizer
   to the minimum necessary to properly release its associated resource.

   Any error while running a finalizer generates a warning; the error is not
   propagated.

  2.5.4 – Weak Tables

   A weak table is a table whose elements are weak references. A weak
   reference is ignored by the garbage collector. In other words, if the only
   references to an object are weak references, then the garbage collector
   will collect that object.

   A weak table can have weak keys, weak values, or both. A table with weak
   values allows the collection of its values, but prevents the collection of
   its keys. A table with both weak keys and weak values allows the
   collection of both keys and values. In any case, if either the key or the
   value is collected, the whole pair is removed from the table. The weakness
   of a table is controlled by the __mode field of its metatable. This
   metavalue, if present, must be one of the following strings: "k", for a
   table with weak keys; "v", for a table with weak values; or "kv", for a
   table with both weak keys and values.

   A table with weak keys and strong values is also called an ephemeron
   table. In an ephemeron table, a value is considered reachable only if its
   key is reachable. In particular, if the only reference to a key comes
   through its value, the pair is removed.

   Any change in the weakness of a table may take effect only at the next
   collect cycle. In particular, if you change the weakness to a stronger
   mode, Lua may still collect some items from that table before the change
   takes effect.

   Only objects that have an explicit construction are removed from weak
   tables. Values, such as numbers and light C functions, are not subject to
   garbage collection, and therefore are not removed from weak tables (unless
   their associated values are collected). Although strings are subject to
   garbage collection, they do not have an explicit construction and their
   equality is by value; they behave more like values than like objects.
   Therefore, they are not removed from weak tables.

   Resurrected objects (that is, objects being finalized and objects
   accessible only through objects being finalized) have a special behavior
   in weak tables. They are removed from weak values before running their
   finalizers, but are removed from weak keys only in the next collection
   after running their finalizers, when such objects are actually freed. This
   behavior allows the finalizer to access properties associated with the
   object through weak tables.

   If a weak table is among the resurrected objects in a collection cycle, it
   may not be properly cleared until the next cycle.

2.6 – Coroutines

   Lua supports coroutines, also called collaborative multithreading. A
   coroutine in Lua represents an independent thread of execution. Unlike
   threads in multithread systems, however, a coroutine only suspends its
   execution by explicitly calling a yield function.

   You create a coroutine by calling coroutine.create. Its sole argument is a
   function that is the main function of the coroutine. The create function
   only creates a new coroutine and returns a handle to it (an object of type
   thread); it does not start the coroutine.

   You execute a coroutine by calling coroutine.resume. When you first call
   coroutine.resume, passing as its first argument a thread returned by
   coroutine.create, the coroutine starts its execution by calling its main
   function. Extra arguments passed to coroutine.resume are passed as
   arguments to that function. After the coroutine starts running, it runs
   until it terminates or yields.

   A coroutine can terminate its execution in two ways: normally, when its
   main function returns (explicitly or implicitly, after the last
   instruction); and abnormally, if there is an unprotected error. In case of
   normal termination, coroutine.resume returns true, plus any values
   returned by the coroutine main function. In case of errors,
   coroutine.resume returns false plus the error object. In this case, the
   coroutine does not unwind its stack, so that it is possible to inspect it
   after the error with the debug API.

   A coroutine yields by calling coroutine.yield. When a coroutine yields,
   the corresponding coroutine.resume returns immediately, even if the yield
   happens inside nested function calls (that is, not in the main function,
   but in a function directly or indirectly called by the main function). In
   the case of a yield, coroutine.resume also returns true, plus any values
   passed to coroutine.yield. The next time you resume the same coroutine, it
   continues its execution from the point where it yielded, with the call to
   coroutine.yield returning any extra arguments passed to coroutine.resume.

   Like coroutine.create, the coroutine.wrap function also creates a
   coroutine, but instead of returning the coroutine itself, it returns a
   function that, when called, resumes the coroutine. Any arguments passed to
   this function go as extra arguments to coroutine.resume. coroutine.wrap
   returns all the values returned by coroutine.resume, except the first one
   (the boolean error code). Unlike coroutine.resume, the function created by
   coroutine.wrap propagates any error to the caller. In this case, the
   function also closes the coroutine (see coroutine.close).

   As an example of how coroutines work, consider the following code:

      function foo (a)
        print("foo", a)
        return coroutine.yield(2*a)
      end
     
      co = coroutine.create(function (a,b)
            print("co-body", a, b)
            local r = foo(a+1)
            print("co-body", r)
            local r, s = coroutine.yield(a+b, a-b)
            print("co-body", r, s)
            return b, "end"
      end)
     
      print("main", coroutine.resume(co, 1, 10))
      print("main", coroutine.resume(co, "r"))
      print("main", coroutine.resume(co, "x", "y"))
      print("main", coroutine.resume(co, "x", "y"))

   When you run it, it produces the following output:

      co-body 1       10
      foo     2
      main    true    4
      co-body r
      main    true    11      -9
      co-body x       y
      main    true    10      end
      main    false   cannot resume dead coroutine

   You can also create and manipulate coroutines through the C API: see
   functions lua_newthread, lua_resume, and lua_yield.

                                3 – The Language

   This section describes the lexis, the syntax, and the semantics of Lua. In
   other words, this section describes which tokens are valid, how they can
   be combined, and what their combinations mean.

   Language constructs will be explained using the usual extended BNF
   notation, in which {a} means 0 or more a's, and [a] means an optional a.
   Non-terminals are shown like non-terminal, keywords are shown like kword,
   and other terminal symbols are shown like ‘=’. The complete syntax of Lua
   can be found in §9 at the end of this manual.

3.1 – Lexical Conventions

   Lua is a free-form language. It ignores spaces and comments between
   lexical elements (tokens), except as delimiters between two tokens. In
   source code, Lua recognizes as spaces the standard ASCII whitespace
   characters space, form feed, newline, carriage return, horizontal tab, and
   vertical tab.

   Names (also called identifiers) in Lua can be any string of Latin letters,
   Arabic-Indic digits, and underscores, not beginning with a digit and not
   being a reserved word. Identifiers are used to name variables, table
   fields, and labels.

   The following keywords are reserved and cannot be used as names:

      and       break     do        else      elseif    end
      false     for       function  goto      if        in
      local     nil       not       or        repeat    return
      then      true      until     while

   Lua is a case-sensitive language: and is a reserved word, but And and AND
   are two different, valid names. As a convention, programs should avoid
   creating names that start with an underscore followed by one or more
   uppercase letters (such as _VERSION).

   The following strings denote other tokens:

      +     -     *     /     %     ^     #
      &     ~     |     <<    >>    //
      ==    ~=    <=    >=    <     >     =
      (     )     {     }     [     ]     ::
      ;     :     ,     .     ..    ...

   A short literal string can be delimited by matching single or double
   quotes, and can contain the following C-like escape sequences: '\a'
   (bell), '\b' (backspace), '\f' (form feed), '\n' (newline), '\r' (carriage
   return), '\t' (horizontal tab), '\v' (vertical tab), '\\' (backslash),
   '\"' (quotation mark [double quote]), and '\'' (apostrophe [single
   quote]). A backslash followed by a line break results in a newline in the
   string. The escape sequence '\z' skips the following span of whitespace
   characters, including line breaks; it is particularly useful to break and
   indent a long literal string into multiple lines without adding the
   newlines and spaces into the string contents. A short literal string
   cannot contain unescaped line breaks nor escapes not forming a valid
   escape sequence.

   We can specify any byte in a short literal string, including embedded
   zeros, by its numeric value. This can be done with the escape sequence
   \xXX, where XX is a sequence of exactly two hexadecimal digits, or with
   the escape sequence \ddd, where ddd is a sequence of up to three decimal
   digits. (Note that if a decimal escape sequence is to be followed by a
   digit, it must be expressed using exactly three digits.)

   The UTF-8 encoding of a Unicode character can be inserted in a literal
   string with the escape sequence \u{XXX} (with mandatory enclosing braces),
   where XXX is a sequence of one or more hexadecimal digits representing the
   character code point. This code point can be any value less than 2^31.
   (Lua uses the original UTF-8 specification here, which is not restricted
   to valid Unicode code points.)

   Literal strings can also be defined using a long format enclosed by long
   brackets. We define an opening long bracket of level n as an opening
   square bracket followed by n equal signs followed by another opening
   square bracket. So, an opening long bracket of level 0 is written as [[,
   an opening long bracket of level 1 is written as [=[, and so on. A closing
   long bracket is defined similarly; for instance, a closing long bracket of
   level 4 is written as ]====]. A long literal starts with an opening long
   bracket of any level and ends at the first closing long bracket of the
   same level. It can contain any text except a closing bracket of the same
   level. Literals in this bracketed form can run for several lines, do not
   interpret any escape sequences, and ignore long brackets of any other
   level. Any kind of end-of-line sequence (carriage return, newline,
   carriage return followed by newline, or newline followed by carriage
   return) is converted to a simple newline. When the opening long bracket is
   immediately followed by a newline, the newline is not included in the
   string.

   As an example, in a system using ASCII (in which 'a' is coded as 97,
   newline is coded as 10, and '1' is coded as 49), the five literal strings
   below denote the same string:

      a = 'alo\n123"'
      a = "alo\n123\""
      a = '\97lo\10\04923"'
      a = [[alo
      123"]]
      a = [==[
      alo
      123"]==]

   Any byte in a literal string not explicitly affected by the previous rules
   represents itself. However, Lua opens files for parsing in text mode, and
   the system's file functions may have problems with some control
   characters. So, it is safer to represent binary data as a quoted literal
   with explicit escape sequences for the non-text characters.

   A numeric constant (or numeral) can be written with an optional fractional
   part and an optional decimal exponent, marked by a letter 'e' or 'E'. Lua
   also accepts hexadecimal constants, which start with 0x or 0X. Hexadecimal
   constants also accept an optional fractional part plus an optional binary
   exponent, marked by a letter 'p' or 'P'.

   A numeric constant with a radix point or an exponent denotes a float;
   otherwise, if its value fits in an integer or it is a hexadecimal
   constant, it denotes an integer; otherwise (that is, a decimal integer
   numeral that overflows), it denotes a float. Hexadecimal numerals with
   neither a radix point nor an exponent always denote an integer value; if
   the value overflows, it wraps around to fit into a valid integer.

   Examples of valid integer constants are

      3   345   0xff   0xBEBADA

   Examples of valid float constants are

      3.0     3.1416     314.16e-2     0.31416E1     34e1
      0x0.1E  0xA23p-4   0X1.921FB54442D18P+1

   A comment starts with a double hyphen (--) anywhere outside a string. If
   the text immediately after -- is not an opening long bracket, the comment
   is a short comment, which runs until the end of the line. Otherwise, it is
   a long comment, which runs until the corresponding closing long bracket.

3.2 – Variables

   Variables are places that store values. There are three kinds of variables
   in Lua: global variables, local variables, and table fields.

   A single name can denote a global variable or a local variable (or a
   function's formal parameter, which is a particular kind of local
   variable):

         var ::= Name

   Name denotes identifiers (see §3.1).

   Any variable name is assumed to be global unless explicitly declared as a
   local (see §3.3.7). Local variables are lexically scoped: local variables
   can be freely accessed by functions defined inside their scope (see §3.5).

   Before the first assignment to a variable, its value is nil.

   Square brackets are used to index a table:

         var ::= prefixexp ‘[’ exp ‘]’

   The meaning of accesses to table fields can be changed via metatables (see
   §2.4).

   The syntax var.Name is just syntactic sugar for var["Name"]:

         var ::= prefixexp ‘.’ Name

   An access to a global variable x is equivalent to _ENV.x. Due to the way
   that chunks are compiled, the variable _ENV itself is never global (see
   §2.2).

3.3 – Statements

   Lua supports an almost conventional set of statements, similar to those in
   other conventional languages. This set includes blocks, assignments,
   control structures, function calls, and variable declarations.

  3.3.1 – Blocks

   A block is a list of statements, which are executed sequentially:

         block ::= {stat}

   Lua has empty statements that allow you to separate statements with
   semicolons, start a block with a semicolon or write two semicolons in
   sequence:

         stat ::= ‘;’

   Both function calls and assignments can start with an open parenthesis.
   This possibility leads to an ambiguity in Lua's grammar. Consider the
   following fragment:

      a = b + c
      (print or io.write)('done')

   The grammar could see this fragment in two ways:

      a = b + c(print or io.write)('done')
     
      a = b + c; (print or io.write)('done')

   The current parser always sees such constructions in the first way,
   interpreting the open parenthesis as the start of the arguments to a call.
   To avoid this ambiguity, it is a good practice to always precede with a
   semicolon statements that start with a parenthesis:

      ;(print or io.write)('done')

   A block can be explicitly delimited to produce a single statement:

         stat ::= do block end

   Explicit blocks are useful to control the scope of variable declarations.
   Explicit blocks are also sometimes used to add a return statement in the
   middle of another block (see §3.3.4).

  3.3.2 – Chunks

   The unit of compilation of Lua is called a chunk. Syntactically, a chunk
   is simply a block:

         chunk ::= block

   Lua handles a chunk as the body of an anonymous function with a variable
   number of arguments (see §3.4.11). As such, chunks can define local
   variables, receive arguments, and return values. Moreover, such anonymous
   function is compiled as in the scope of an external local variable called
   _ENV (see §2.2). The resulting function always has _ENV as its only
   external variable, even if it does not use that variable.

   A chunk can be stored in a file or in a string inside the host program. To
   execute a chunk, Lua first loads it, precompiling the chunk's code into
   instructions for a virtual machine, and then Lua executes the compiled
   code with an interpreter for the virtual machine.

   Chunks can also be precompiled into binary form; see the program luac and
   the function string.dump for details. Programs in source and compiled
   forms are interchangeable; Lua automatically detects the file type and
   acts accordingly (see load).

  3.3.3 – Assignment

   Lua allows multiple assignments. Therefore, the syntax for assignment
   defines a list of variables on the left side and a list of expressions on
   the right side. The elements in both lists are separated by commas:

         stat ::= varlist ‘=’ explist
         varlist ::= var {‘,’ var}
         explist ::= exp {‘,’ exp}

   Expressions are discussed in §3.4.

   Before the assignment, the list of values is adjusted to the length of the
   list of variables. If there are more values than needed, the excess values
   are thrown away. If there are fewer values than needed, the list is
   extended with nil's. If the list of expressions ends with a function call,
   then all values returned by that call enter the list of values, before the
   adjustment (except when the call is enclosed in parentheses; see §3.4).

   If a variable is both assigned and read inside a multiple assignment, Lua
   ensures all reads get the value of the variable before the assignment.
   Thus the code

      i = 3
      i, a[i] = i+1, 20

   sets a[3] to 20, without affecting a[4] because the i in a[i] is evaluated
   (to 3) before it is assigned 4. Similarly, the line

      x, y = y, x

   exchanges the values of x and y, and

      x, y, z = y, z, x

   cyclically permutes the values of x, y, and z.

   Note that this guarantee covers only accesses syntactically inside the
   assignment statement. If a function or a metamethod called during the
   assignment changes the value of a variable, Lua gives no guarantees about
   the order of that access.

   An assignment to a global name x = val is equivalent to the assignment
   _ENV.x = val (see §2.2).

   The meaning of assignments to table fields and global variables (which are
   actually table fields, too) can be changed via metatables (see §2.4).

  3.3.4 – Control Structures

   The control structures if, while, and repeat have the usual meaning and
   familiar syntax:

         stat ::= while exp do block end
         stat ::= repeat block until exp
         stat ::= if exp then block {elseif exp then block} [else block] end

   Lua also has a for statement, in two flavors (see §3.3.5).

   The condition expression of a control structure can return any value. Both
   false and nil test false. All values different from nil and false test
   true. In particular, the number 0 and the empty string also test true.

   In the repeat–until loop, the inner block does not end at the until
   keyword, but only after the condition. So, the condition can refer to
   local variables declared inside the loop block.

   The goto statement transfers the program control to a label. For
   syntactical reasons, labels in Lua are considered statements too:

         stat ::= goto Name
         stat ::= label
         label ::= ‘::’ Name ‘::’

   A label is visible in the entire block where it is defined, except inside
   nested functions. A goto may jump to any visible label as long as it does
   not enter into the scope of a local variable. A label should not be
   declared where a label with the same name is visible, even if this other
   label has been declared in an enclosing block.

   Labels and empty statements are called void statements, as they perform no
   actions.

   The break statement terminates the execution of a while, repeat, or for
   loop, skipping to the next statement after the loop:

         stat ::= break

   A break ends the innermost enclosing loop.

   The return statement is used to return values from a function or a chunk
   (which is handled as an anonymous function). Functions can return more
   than one value, so the syntax for the return statement is

         stat ::= return [explist] [‘;’]

   The return statement can only be written as the last statement of a block.
   If it is necessary to return in the middle of a block, then an explicit
   inner block can be used, as in the idiom do return end, because now return
   is the last statement in its (inner) block.

  3.3.5 – For Statement

   The for statement has two forms: one numerical and one generic.

    The numerical for loop

   The numerical for loop repeats a block of code while a control variable
   goes through an arithmetic progression. It has the following syntax:

         stat ::= for Name ‘=’ exp ‘,’ exp [‘,’ exp] do block end

   The given identifier (Name) defines the control variable, which is a new
   variable local to the loop body (block).

   The loop starts by evaluating once the three control expressions. Their
   values are called respectively the initial value, the limit, and the step.
   If the step is absent, it defaults to 1.

   If both the initial value and the step are integers, the loop is done with
   integers; note that the limit may not be an integer. Otherwise, the three
   values are converted to floats and the loop is done with floats. Beware of
   floating-point accuracy in this case.

   After that initialization, the loop body is repeated with the value of the
   control variable going through an arithmetic progression, starting at the
   initial value, with a common difference given by the step. A negative step
   makes a decreasing sequence; a step equal to zero raises an error. The
   loop continues while the value is less than or equal to the limit (greater
   than or equal to for a negative step). If the initial value is already
   greater than the limit (or less than, if the step is negative), the body
   is not executed.

   For integer loops, the control variable never wraps around; instead, the
   loop ends in case of an overflow.

   You should not change the value of the control variable during the loop.
   If you need its value after the loop, assign it to another variable before
   exiting the loop.

    The generic for loop

   The generic for statement works over functions, called iterators. On each
   iteration, the iterator function is called to produce a new value,
   stopping when this new value is nil. The generic for loop has the
   following syntax:

         stat ::= for namelist in explist do block end
         namelist ::= Name {‘,’ Name}

   A for statement like

      for var_1, ···, var_n in explist do body end

   works as follows.

   The names var_i declare loop variables local to the loop body. The first
   of these variables is the control variable.

   The loop starts by evaluating explist to produce four values: an iterator
   function, a state, an initial value for the control variable, and a
   closing value.

   Then, at each iteration, Lua calls the iterator function with two
   arguments: the state and the control variable. The results from this call
   are then assigned to the loop variables, following the rules of multiple
   assignments (see §3.3.3). If the control variable becomes nil, the loop
   terminates. Otherwise, the body is executed and the loop goes to the next
   iteration.

   The closing value behaves like a to-be-closed variable (see §3.3.8), which
   can be used to release resources when the loop ends. Otherwise, it does
   not interfere with the loop.

   You should not change the value of the control variable during the loop.

  3.3.6 – Function Calls as Statements

   To allow possible side-effects, function calls can be executed as
   statements:

         stat ::= functioncall

   In this case, all returned values are thrown away. Function calls are
   explained in §3.4.10.

  3.3.7 – Local Declarations

   Local variables can be declared anywhere inside a block. The declaration
   can include an initialization:

         stat ::= local attnamelist [‘=’ explist]
         attnamelist ::=  Name attrib {‘,’ Name attrib}

   If present, an initial assignment has the same semantics of a multiple
   assignment (see §3.3.3). Otherwise, all variables are initialized with
   nil.

   Each variable name may be postfixed by an attribute (a name between angle
   brackets):

         attrib ::= [‘<’ Name ‘>’]

   There are two possible attributes: const, which declares a constant
   variable, that is, a variable that cannot be assigned to after its
   initialization; and close, which declares a to-be-closed variable (see
   §3.3.8). A list of variables can contain at most one to-be-closed
   variable.

   A chunk is also a block (see §3.3.2), and so local variables can be
   declared in a chunk outside any explicit block.

   The visibility rules for local variables are explained in §3.5.

  3.3.8 – To-be-closed Variables

   A to-be-closed variable behaves like a constant local variable, except
   that its value is closed whenever the variable goes out of scope,
   including normal block termination, exiting its block by
   break/goto/return, or exiting by an error.

   Here, to close a value means to call its __close metamethod. When calling
   the metamethod, the value itself is passed as the first argument and the
   error object that caused the exit (if any) is passed as a second argument;
   if there was no error, the second argument is nil.

   The value assigned to a to-be-closed variable must have a __close
   metamethod or be a false value. (nil and false are ignored as to-be-closed
   values.)

   If several to-be-closed variables go out of scope at the same event, they
   are closed in the reverse order that they were declared.

   If there is any error while running a closing method, that error is
   handled like an error in the regular code where the variable was defined.
   After an error, the other pending closing methods will still be called.

   If a coroutine yields and is never resumed again, some variables may never
   go out of scope, and therefore they will never be closed. (These variables
   are the ones created inside the coroutine and in scope at the point where
   the coroutine yielded.) Similarly, if a coroutine ends with an error, it
   does not unwind its stack, so it does not close any variable. In both
   cases, you can either use finalizers or call coroutine.close to close the
   variables. However, if the coroutine was created through coroutine.wrap,
   then its corresponding function will close the coroutine in case of
   errors.

3.4 – Expressions

   The basic expressions in Lua are the following:

         exp ::= prefixexp
         exp ::= nil | false | true
         exp ::= Numeral
         exp ::= LiteralString
         exp ::= functiondef
         exp ::= tableconstructor
         exp ::= ‘...’
         exp ::= exp binop exp
         exp ::= unop exp
         prefixexp ::= var | functioncall | ‘(’ exp ‘)’

   Numerals and literal strings are explained in §3.1; variables are
   explained in §3.2; function definitions are explained in §3.4.11; function
   calls are explained in §3.4.10; table constructors are explained in
   §3.4.9. Vararg expressions, denoted by three dots ('...'), can only be
   used when directly inside a vararg function; they are explained in
   §3.4.11.

   Binary operators comprise arithmetic operators (see §3.4.1), bitwise
   operators (see §3.4.2), relational operators (see §3.4.4), logical
   operators (see §3.4.5), and the concatenation operator (see §3.4.6). Unary
   operators comprise the unary minus (see §3.4.1), the unary bitwise NOT
   (see §3.4.2), the unary logical not (see §3.4.5), and the unary length
   operator (see §3.4.7).

   Both function calls and vararg expressions can result in multiple values.
   If a function call is used as a statement (see §3.3.6), then its return
   list is adjusted to zero elements, thus discarding all returned values. If
   an expression is used as the last (or the only) element of a list of
   expressions, then no adjustment is made (unless the expression is enclosed
   in parentheses). In all other contexts, Lua adjusts the result list to one
   element, either discarding all values except the first one or adding a
   single nil if there are no values.

   Here are some examples:

      f()                -- adjusted to 0 results
      g(f(), x)          -- f() is adjusted to 1 result
      g(x, f())          -- g gets x plus all results from f()
      a,b,c = f(), x     -- f() is adjusted to 1 result (c gets nil)
      a,b = ...          -- a gets the first vararg argument, b gets
                         -- the second (both a and b can get nil if there
                         -- is no corresponding vararg argument)
     
      a,b,c = x, f()     -- f() is adjusted to 2 results
      a,b,c = f()        -- f() is adjusted to 3 results
      return f()         -- returns all results from f()
      return ...         -- returns all received vararg arguments
      return x,y,f()     -- returns x, y, and all results from f()
      {f()}              -- creates a list with all results from f()
      {...}              -- creates a list with all vararg arguments
      {f(), nil}         -- f() is adjusted to 1 result

   Any expression enclosed in parentheses always results in only one value.
   Thus, (f(x,y,z)) is always a single value, even if f returns several
   values. (The value of (f(x,y,z)) is the first value returned by f or nil
   if f does not return any values.)

  3.4.1 – Arithmetic Operators

   Lua supports the following arithmetic operators:
     * +: addition
     * -: subtraction
     * *: multiplication
     * /: float division
     * //: floor division
     * %: modulo
     * ^: exponentiation
     * -: unary minus

   With the exception of exponentiation and float division, the arithmetic
   operators work as follows: If both operands are integers, the operation is
   performed over integers and the result is an integer. Otherwise, if both
   operands are numbers, then they are converted to floats, the operation is
   performed following the machine's rules for floating-point arithmetic
   (usually the IEEE 754 standard), and the result is a float. (The string
   library coerces strings to numbers in arithmetic operations; see §3.4.3
   for details.)

   Exponentiation and float division (/) always convert their operands to
   floats and the result is always a float. Exponentiation uses the ISO C
   function pow, so that it works for non-integer exponents too.

   Floor division (//) is a division that rounds the quotient towards minus
   infinity, resulting in the floor of the division of its operands.

   Modulo is defined as the remainder of a division that rounds the quotient
   towards minus infinity (floor division).

   In case of overflows in integer arithmetic, all operations wrap around.

  3.4.2 – Bitwise Operators

   Lua supports the following bitwise operators:
     * &: bitwise AND
     * |: bitwise OR
     * ~: bitwise exclusive OR
     * >>: right shift
     * <<: left shift
     * ~: unary bitwise NOT

   All bitwise operations convert its operands to integers (see §3.4.3),
   operate on all bits of those integers, and result in an integer.

   Both right and left shifts fill the vacant bits with zeros. Negative
   displacements shift to the other direction; displacements with absolute
   values equal to or higher than the number of bits in an integer result in
   zero (as all bits are shifted out).

  3.4.3 – Coercions and Conversions

   Lua provides some automatic conversions between some types and
   representations at run time. Bitwise operators always convert float
   operands to integers. Exponentiation and float division always convert
   integer operands to floats. All other arithmetic operations applied to
   mixed numbers (integers and floats) convert the integer operand to a
   float. The C API also converts both integers to floats and floats to
   integers, as needed. Moreover, string concatenation accepts numbers as
   arguments, besides strings.

   In a conversion from integer to float, if the integer value has an exact
   representation as a float, that is the result. Otherwise, the conversion
   gets the nearest higher or the nearest lower representable value. This
   kind of conversion never fails.

   The conversion from float to integer checks whether the float has an exact
   representation as an integer (that is, the float has an integral value and
   it is in the range of integer representation). If it does, that
   representation is the result. Otherwise, the conversion fails.

   Several places in Lua coerce strings to numbers when necessary. In
   particular, the string library sets metamethods that try to coerce strings
   to numbers in all arithmetic operations. If the conversion fails, the
   library calls the metamethod of the other operand (if present) or it
   raises an error. Note that bitwise operators do not do this coercion.

   Nonetheless, it is always a good practice not to rely on these implicit
   coercions, as they are not always applied; in particular, "1"==1 is false
   and "1"<1 raises an error (see §3.4.4). These coercions exist mainly for
   compatibility and may be removed in future versions of the language.

   A string is converted to an integer or a float following its syntax and
   the rules of the Lua lexer. The string may have also leading and trailing
   whitespaces and a sign. All conversions from strings to numbers accept
   both a dot and the current locale mark as the radix character. (The Lua
   lexer, however, accepts only a dot.) If the string is not a valid numeral,
   the conversion fails. If necessary, the result of this first step is then
   converted to a specific number subtype following the previous rules for
   conversions between floats and integers.

   The conversion from numbers to strings uses a non-specified human-readable
   format. To convert numbers to strings in any specific way, use the
   function string.format.

  3.4.4 – Relational Operators

   Lua supports the following relational operators:
     * ==: equality
     * ~=: inequality
     * <: less than
     * >: greater than
     * <=: less or equal
     * >=: greater or equal

   These operators always result in false or true.

   Equality (==) first compares the type of its operands. If the types are
   different, then the result is false. Otherwise, the values of the operands
   are compared. Strings are equal if they have the same byte content.
   Numbers are equal if they denote the same mathematical value.

   Tables, userdata, and threads are compared by reference: two objects are
   considered equal only if they are the same object. Every time you create a
   new object (a table, a userdata, or a thread), this new object is
   different from any previously existing object. A function is always equal
   to itself. Functions with any detectable difference (different behavior,
   different definition) are always different. Functions created at different
   times but with no detectable differences may be classified as equal or not
   (depending on internal caching details).

   You can change the way that Lua compares tables and userdata by using the
   __eq metamethod (see §2.4).

   Equality comparisons do not convert strings to numbers or vice versa.
   Thus, "0"==0 evaluates to false, and t[0] and t["0"] denote different
   entries in a table.

   The operator ~= is exactly the negation of equality (==).

   The order operators work as follows. If both arguments are numbers, then
   they are compared according to their mathematical values, regardless of
   their subtypes. Otherwise, if both arguments are strings, then their
   values are compared according to the current locale. Otherwise, Lua tries
   to call the __lt or the __le metamethod (see §2.4). A comparison a > b is
   translated to b < a and a >= b is translated to b <= a.

   Following the IEEE 754 standard, the special value NaN is considered
   neither less than, nor equal to, nor greater than any value, including
   itself.

  3.4.5 – Logical Operators

   The logical operators in Lua are and, or, and not. Like the control
   structures (see §3.3.4), all logical operators consider both false and nil
   as false and anything else as true.

   The negation operator not always returns false or true. The conjunction
   operator and returns its first argument if this value is false or nil;
   otherwise, and returns its second argument. The disjunction operator or
   returns its first argument if this value is different from nil and false;
   otherwise, or returns its second argument. Both and and or use
   short-circuit evaluation; that is, the second operand is evaluated only if
   necessary. Here are some examples:

      10 or 20            --> 10
      10 or error()       --> 10
      nil or "a"          --> "a"
      nil and 10          --> nil
      false and error()   --> false
      false and nil       --> false
      false or nil        --> nil
      10 and 20           --> 20

  3.4.6 – Concatenation

   The string concatenation operator in Lua is denoted by two dots ('..'). If
   both operands are strings or numbers, then the numbers are converted to
   strings in a non-specified format (see §3.4.3). Otherwise, the __concat
   metamethod is called (see §2.4).

  3.4.7 – The Length Operator

   The length operator is denoted by the unary prefix operator #.

   The length of a string is its number of bytes. (That is the usual meaning
   of string length when each character is one byte.)

   The length operator applied on a table returns a border in that table. A
   border in a table t is any non-negative integer that satisfies the
   following condition:

      (border == 0 or t[border] ~= nil) and
      (t[border + 1] == nil or border == math.maxinteger)

   In words, a border is any positive integer index present in the table that
   is followed by an absent index, plus two limit cases: zero, when index 1
   is absent; and the maximum value for an integer, when that index is
   present. Note that keys that are not positive integers do not interfere
   with borders.

   A table with exactly one border is called a sequence. For instance, the
   table {10, 20, 30, 40, 50} is a sequence, as it has only one border (5).
   The table {10, 20, 30, nil, 50} has two borders (3 and 5), and therefore
   it is not a sequence. (The nil at index 4 is called a hole.) The table
   {nil, 20, 30, nil, nil, 60, nil} has three borders (0, 3, and 6), so it is
   not a sequence, too. The table {} is a sequence with border 0.

   When t is a sequence, #t returns its only border, which corresponds to the
   intuitive notion of the length of the sequence. When t is not a sequence,
   #t can return any of its borders. (The exact one depends on details of the
   internal representation of the table, which in turn can depend on how the
   table was populated and the memory addresses of its non-numeric keys.)

   The computation of the length of a table has a guaranteed worst time of
   O(log n), where n is the largest integer key in the table.

   A program can modify the behavior of the length operator for any value but
   strings through the __len metamethod (see §2.4).

  3.4.8 – Precedence

   Operator precedence in Lua follows the table below, from lower to higher
   priority:

      or
      and
      <     >     <=    >=    ~=    ==
      |
      ~
      &
      <<    >>
      ..
      +     -
      *     /     //    %
      unary operators (not   #     -     ~)
      ^

   As usual, you can use parentheses to change the precedences of an
   expression. The concatenation ('..') and exponentiation ('^') operators
   are right associative. All other binary operators are left associative.

  3.4.9 – Table Constructors

   Table constructors are expressions that create tables. Every time a
   constructor is evaluated, a new table is created. A constructor can be
   used to create an empty table or to create a table and initialize some of
   its fields. The general syntax for constructors is

         tableconstructor ::= ‘{’ [fieldlist] ‘}’
         fieldlist ::= field {fieldsep field} [fieldsep]
         field ::= ‘[’ exp ‘]’ ‘=’ exp | Name ‘=’ exp | exp
         fieldsep ::= ‘,’ | ‘;’

   Each field of the form [exp1] = exp2 adds to the new table an entry with
   key exp1 and value exp2. A field of the form name = exp is equivalent to
   ["name"] = exp. Fields of the form exp are equivalent to [i] = exp, where
   i are consecutive integers starting with 1; fields in the other formats do
   not affect this counting. For example,

      a = { [f(1)] = g; "x", "y"; x = 1, f(x), [30] = 23; 45 }

   is equivalent to

      do
        local t = {}
        t[f(1)] = g
        t[1] = "x"         -- 1st exp
        t[2] = "y"         -- 2nd exp
        t.x = 1            -- t["x"] = 1
        t[3] = f(x)        -- 3rd exp
        t[30] = 23
        t[4] = 45          -- 4th exp
        a = t
      end

   The order of the assignments in a constructor is undefined. (This order
   would be relevant only when there are repeated keys.)

   If the last field in the list has the form exp and the expression is a
   function call or a vararg expression, then all values returned by this
   expression enter the list consecutively (see §3.4.10).

   The field list can have an optional trailing separator, as a convenience
   for machine-generated code.

  3.4.10 – Function Calls

   A function call in Lua has the following syntax:

         functioncall ::= prefixexp args

   In a function call, first prefixexp and args are evaluated. If the value
   of prefixexp has type function, then this function is called with the
   given arguments. Otherwise, if present, the prefixexp __call metamethod is
   called: its first argument is the value of prefixexp, followed by the
   original call arguments (see §2.4).

   The form

         functioncall ::= prefixexp ‘:’ Name args

   can be used to emulate methods. A call v:name(args) is syntactic sugar for
   v.name(v,args), except that v is evaluated only once.

   Arguments have the following syntax:

         args ::= ‘(’ [explist] ‘)’
         args ::= tableconstructor
         args ::= LiteralString

   All argument expressions are evaluated before the call. A call of the form
   f{fields} is syntactic sugar for f({fields}); that is, the argument list
   is a single new table. A call of the form f'string' (or f"string" or
   f[[string]]) is syntactic sugar for f('string'); that is, the argument
   list is a single literal string.

   A call of the form return functioncall not in the scope of a to-be-closed
   variable is called a tail call. Lua implements proper tail calls (or
   proper tail recursion): in a tail call, the called function reuses the
   stack entry of the calling function. Therefore, there is no limit on the
   number of nested tail calls that a program can execute. However, a tail
   call erases any debug information about the calling function. Note that a
   tail call only happens with a particular syntax, where the return has one
   single function call as argument, and it is outside the scope of any
   to-be-closed variable. This syntax makes the calling function return
   exactly the returns of the called function, without any intervening
   action. So, none of the following examples are tail calls:

      return (f(x))        -- results adjusted to 1
      return 2 * f(x)      -- result multiplied by 2
      return x, f(x)       -- additional results
      f(x); return         -- results discarded
      return x or f(x)     -- results adjusted to 1

  3.4.11 – Function Definitions

   The syntax for function definition is

         functiondef ::= function funcbody
         funcbody ::= ‘(’ [parlist] ‘)’ block end

   The following syntactic sugar simplifies function definitions:

         stat ::= function funcname funcbody
         stat ::= local function Name funcbody
         funcname ::= Name {‘.’ Name} [‘:’ Name]

   The statement

      function f () body end

   translates to

      f = function () body end

   The statement

      function t.a.b.c.f () body end

   translates to

      t.a.b.c.f = function () body end

   The statement

      local function f () body end

   translates to

      local f; f = function () body end

   not to

      local f = function () body end

   (This only makes a difference when the body of the function contains
   references to f.)

   A function definition is an executable expression, whose value has type
   function. When Lua precompiles a chunk, all its function bodies are
   precompiled too, but they are not created yet. Then, whenever Lua executes
   the function definition, the function is instantiated (or closed). This
   function instance, or closure, is the final value of the expression.

   Parameters act as local variables that are initialized with the argument
   values:

         parlist ::= namelist [‘,’ ‘...’] | ‘...’

   When a Lua function is called, it adjusts its list of arguments to the
   length of its list of parameters, unless the function is a vararg
   function, which is indicated by three dots ('...') at the end of its
   parameter list. A vararg function does not adjust its argument list;
   instead, it collects all extra arguments and supplies them to the function
   through a vararg expression, which is also written as three dots. The
   value of this expression is a list of all actual extra arguments, similar
   to a function with multiple results. If a vararg expression is used inside
   another expression or in the middle of a list of expressions, then its
   return list is adjusted to one element. If the expression is used as the
   last element of a list of expressions, then no adjustment is made (unless
   that last expression is enclosed in parentheses).

   As an example, consider the following definitions:

      function f(a, b) end
      function g(a, b, ...) end
      function r() return 1,2,3 end

   Then, we have the following mapping from arguments to parameters and to
   the vararg expression:

      CALL             PARAMETERS
     
      f(3)             a=3, b=nil
      f(3, 4)          a=3, b=4
      f(3, 4, 5)       a=3, b=4
      f(r(), 10)       a=1, b=10
      f(r())           a=1, b=2
     
      g(3)             a=3, b=nil, ... -->  (nothing)
      g(3, 4)          a=3, b=4,   ... -->  (nothing)
      g(3, 4, 5, 8)    a=3, b=4,   ... -->  5  8
      g(5, r())        a=5, b=1,   ... -->  2  3

   Results are returned using the return statement (see §3.3.4). If control
   reaches the end of a function without encountering a return statement,
   then the function returns with no results.

   There is a system-dependent limit on the number of values that a function
   may return. This limit is guaranteed to be greater than 1000.

   The colon syntax is used to emulate methods, adding an implicit extra
   parameter self to the function. Thus, the statement

      function t.a.b.c:f (params) body end

   is syntactic sugar for

      t.a.b.c.f = function (self, params) body end

3.5 – Visibility Rules

   Lua is a lexically scoped language. The scope of a local variable begins
   at the first statement after its declaration and lasts until the last
   non-void statement of the innermost block that includes the declaration.
   Consider the following example:

      x = 10                -- global variable
      do                    -- new block
        local x = x         -- new 'x', with value 10
        print(x)            --> 10
        x = x+1
        do                  -- another block
          local x = x+1     -- another 'x'
          print(x)          --> 12
        end
        print(x)            --> 11
      end
      print(x)              --> 10  (the global one)

   Notice that, in a declaration like local x = x, the new x being declared
   is not in scope yet, and so the second x refers to the outside variable.

   Because of the lexical scoping rules, local variables can be freely
   accessed by functions defined inside their scope. A local variable used by
   an inner function is called an upvalue (or external local variable, or
   simply external variable) inside the inner function.

   Notice that each execution of a local statement defines new local
   variables. Consider the following example:

      a = {}
      local x = 20
      for i = 1, 10 do
        local y = 0
        a[i] = function () y = y + 1; return x + y end
      end

   The loop creates ten closures (that is, ten instances of the anonymous
   function). Each of these closures uses a different y variable, while all
   of them share the same x.

                     4 – The Application Program Interface

   This section describes the C API for Lua, that is, the set of C functions
   available to the host program to communicate with Lua. All API functions
   and related types and constants are declared in the header file lua.h.

   Even when we use the term "function", any facility in the API may be
   provided as a macro instead. Except where stated otherwise, all such
   macros use each of their arguments exactly once (except for the first
   argument, which is always a Lua state), and so do not generate any hidden
   side-effects.

   As in most C libraries, the Lua API functions do not check their arguments
   for validity or consistency. However, you can change this behavior by
   compiling Lua with the macro LUA_USE_APICHECK defined.

   The Lua library is fully reentrant: it has no global variables. It keeps
   all information it needs in a dynamic structure, called the Lua state.

   Each Lua state has one or more threads, which correspond to independent,
   cooperative lines of execution. The type lua_State (despite its name)
   refers to a thread. (Indirectly, through the thread, it also refers to the
   Lua state associated to the thread.)

   A pointer to a thread must be passed as the first argument to every
   function in the library, except to lua_newstate, which creates a Lua state
   from scratch and returns a pointer to the main thread in the new state.

4.1 – The Stack

   Lua uses a virtual stack to pass values to and from C. Each element in
   this stack represents a Lua value (nil, number, string, etc.). Functions
   in the API can access this stack through the Lua state parameter that they
   receive.

   Whenever Lua calls C, the called function gets a new stack, which is
   independent of previous stacks and of stacks of C functions that are still
   active. This stack initially contains any arguments to the C function and
   it is where the C function can store temporary Lua values and must push
   its results to be returned to the caller (see lua_CFunction).

   For convenience, most query operations in the API do not follow a strict
   stack discipline. Instead, they can refer to any element in the stack by
   using an index: A positive index represents an absolute stack position,
   starting at 1 as the bottom of the stack; a negative index represents an
   offset relative to the top of the stack. More specifically, if the stack
   has n elements, then index 1 represents the first element (that is, the
   element that was pushed onto the stack first) and index n represents the
   last element; index -1 also represents the last element (that is, the
   element at the top) and index -n represents the first element.

  4.1.1 – Stack Size

   When you interact with the Lua API, you are responsible for ensuring
   consistency. In particular, you are responsible for controlling stack
   overflow. When you call any API function, you must ensure the stack has
   enough room to accommodate the results.

   There is one exception to the above rule: When you call a Lua function
   without a fixed number of results (see lua_call), Lua ensures that the
   stack has enough space for all results. However, it does not ensure any
   extra space. So, before pushing anything on the stack after such a call
   you should use lua_checkstack.

   Whenever Lua calls C, it ensures that the stack has space for at least
   LUA_MINSTACK extra elements; that is, you can safely push up to
   LUA_MINSTACK values into it. LUA_MINSTACK is defined as 20, so that
   usually you do not have to worry about stack space unless your code has
   loops pushing elements onto the stack. Whenever necessary, you can use the
   function lua_checkstack to ensure that the stack has enough space for
   pushing new elements.

  4.1.2 – Valid and Acceptable Indices

   Any function in the API that receives stack indices works only with valid
   indices or acceptable indices.

   A valid index is an index that refers to a position that stores a
   modifiable Lua value. It comprises stack indices between 1 and the stack
   top (1 ≤ abs(index) ≤ top) plus pseudo-indices, which represent some
   positions that are accessible to C code but that are not in the stack.
   Pseudo-indices are used to access the registry (see §4.3) and the upvalues
   of a C function (see §4.2).

   Functions that do not need a specific mutable position, but only a value
   (e.g., query functions), can be called with acceptable indices. An
   acceptable index can be any valid index, but it also can be any positive
   index after the stack top within the space allocated for the stack, that
   is, indices up to the stack size. (Note that 0 is never an acceptable
   index.) Indices to upvalues (see §4.2) greater than the real number of
   upvalues in the current C function are also acceptable (but invalid).
   Except when noted otherwise, functions in the API work with acceptable
   indices.

   Acceptable indices serve to avoid extra tests against the stack top when
   querying the stack. For instance, a C function can query its third
   argument without the need to check whether there is a third argument, that
   is, without the need to check whether 3 is a valid index.

   For functions that can be called with acceptable indices, any non-valid
   index is treated as if it contains a value of a virtual type LUA_TNONE,
   which behaves like a nil value.

  4.1.3 – Pointers to strings

   Several functions in the API return pointers (const char*) to Lua strings
   in the stack. (See lua_pushfstring, lua_pushlstring, lua_pushstring, and
   lua_tolstring. See also luaL_checklstring, luaL_checkstring, and
   luaL_tolstring in the auxiliary library.)

   In general, Lua's garbage collection can free or move internal memory and
   then invalidate pointers to internal strings. To allow a safe use of these
   pointers, The API guarantees that any pointer to a string in a stack index
   is valid while the string value at that index is not removed from the
   stack. (It can be moved to another index, though.) When the index is a
   pseudo-index (referring to an upvalue), the pointer is valid while the
   corresponding call is active and the corresponding upvalue is not
   modified.

   Some functions in the debug interface also return pointers to strings,
   namely lua_getlocal, lua_getupvalue, lua_setlocal, and lua_setupvalue. For
   these functions, the pointer is guaranteed to be valid while the caller
   function is active and the given closure (if one was given) is in the
   stack.

   Except for these guarantees, the garbage collector is free to invalidate
   any pointer to internal strings.

4.2 – C Closures

   When a C function is created, it is possible to associate some values with
   it, thus creating a C closure (see lua_pushcclosure); these values are
   called upvalues and are accessible to the function whenever it is called.

   Whenever a C function is called, its upvalues are located at specific
   pseudo-indices. These pseudo-indices are produced by the macro
   lua_upvalueindex. The first upvalue associated with a function is at index
   lua_upvalueindex(1), and so on. Any access to lua_upvalueindex(n), where n
   is greater than the number of upvalues of the current function (but not
   greater than 256, which is one plus the maximum number of upvalues in a
   closure), produces an acceptable but invalid index.

   A C closure can also change the values of its corresponding upvalues.

4.3 – Registry

   Lua provides a registry, a predefined table that can be used by any C code
   to store whatever Lua values it needs to store. The registry table is
   always accessible at pseudo-index LUA_REGISTRYINDEX. Any C library can
   store data into this table, but it must take care to choose keys that are
   different from those used by other libraries, to avoid collisions.
   Typically, you should use as key a string containing your library name, or
   a light userdata with the address of a C object in your code, or any Lua
   object created by your code. As with variable names, string keys starting
   with an underscore followed by uppercase letters are reserved for Lua.

   The integer keys in the registry are used by the reference mechanism (see
   luaL_ref) and by some predefined values. Therefore, integer keys in the
   registry must not be used for other purposes.

   When you create a new Lua state, its registry comes with some predefined
   values. These predefined values are indexed with integer keys defined as
   constants in lua.h. The following constants are defined:
     * LUA_RIDX_MAINTHREAD: At this index the registry has the main thread of
       the state. (The main thread is the one created together with the
       state.)
     * LUA_RIDX_GLOBALS: At this index the registry has the global
       environment.

4.4 – Error Handling in C

   Internally, Lua uses the C longjmp facility to handle errors. (Lua will
   use exceptions if you compile it as C++; search for LUAI_THROW in the
   source code for details.) When Lua faces any error, such as a memory
   allocation error or a type error, it raises an error; that is, it does a
   long jump. A protected environment uses setjmp to set a recovery point;
   any error jumps to the most recent active recovery point.

   Inside a C function you can raise an error explicitly by calling
   lua_error.

   Most functions in the API can raise an error, for instance due to a memory
   allocation error. The documentation for each function indicates whether it
   can raise errors.

   If an error happens outside any protected environment, Lua calls a panic
   function (see lua_atpanic) and then calls abort, thus exiting the host
   application. Your panic function can avoid this exit by never returning
   (e.g., doing a long jump to your own recovery point outside Lua).

   The panic function, as its name implies, is a mechanism of last resort.
   Programs should avoid it. As a general rule, when a C function is called
   by Lua with a Lua state, it can do whatever it wants on that Lua state, as
   it should be already protected. However, when C code operates on other Lua
   states (e.g., a Lua-state argument to the function, a Lua state stored in
   the registry, or the result of lua_newthread), it should use them only in
   API calls that cannot raise errors.

   The panic function runs as if it were a message handler (see §2.3); in
   particular, the error object is on the top of the stack. However, there is
   no guarantee about stack space. To push anything on the stack, the panic
   function must first check the available space (see §4.1.1).

  4.4.1 – Status Codes

   Several functions that report errors in the API use the following status
   codes to indicate different kinds of errors or other conditions:
     * LUA_OK (0): no errors.
     * LUA_ERRRUN: a runtime error.
     * LUA_ERRMEM: memory allocation error. For such errors, Lua does not
       call the message handler.
     * LUA_ERRERR: error while running the message handler.
     * LUA_ERRSYNTAX: syntax error during precompilation.
     * LUA_YIELD: the thread (coroutine) yields.
     * LUA_ERRFILE: a file-related error; e.g., it cannot open or read the
       file.

   These constants are defined in the header file lua.h.

4.5 – Handling Yields in C

   Internally, Lua uses the C longjmp facility to yield a coroutine.
   Therefore, if a C function foo calls an API function and this API function
   yields (directly or indirectly by calling another function that yields),
   Lua cannot return to foo any more, because the longjmp removes its frame
   from the C stack.

   To avoid this kind of problem, Lua raises an error whenever it tries to
   yield across an API call, except for three functions: lua_yieldk,
   lua_callk, and lua_pcallk. All those functions receive a continuation
   function (as a parameter named k) to continue execution after a yield.

   We need to set some terminology to explain continuations. We have a
   C function called from Lua which we will call the original function. This
   original function then calls one of those three functions in the C API,
   which we will call the callee function, that then yields the current
   thread. This can happen when the callee function is lua_yieldk, or when
   the callee function is either lua_callk or lua_pcallk and the function
   called by them yields.

   Suppose the running thread yields while executing the callee function.
   After the thread resumes, it eventually will finish running the callee
   function. However, the callee function cannot return to the original
   function, because its frame in the C stack was destroyed by the yield.
   Instead, Lua calls a continuation function, which was given as an argument
   to the callee function. As the name implies, the continuation function
   should continue the task of the original function.

   As an illustration, consider the following function:

      int original_function (lua_State *L) {
        ...     /* code 1 */
        status = lua_pcall(L, n, m, h);  /* calls Lua */
        ...     /* code 2 */
      }

   Now we want to allow the Lua code being run by lua_pcall to yield. First,
   we can rewrite our function like here:

      int k (lua_State *L, int status, lua_KContext ctx) {
        ...  /* code 2 */
      }
     
      int original_function (lua_State *L) {
        ...     /* code 1 */
        return k(L, lua_pcall(L, n, m, h), ctx);
      }

   In the above code, the new function k is a continuation function (with
   type lua_KFunction), which should do all the work that the original
   function was doing after calling lua_pcall. Now, we must inform Lua that
   it must call k if the Lua code being executed by lua_pcall gets
   interrupted in some way (errors or yielding), so we rewrite the code as
   here, replacing lua_pcall by lua_pcallk:

      int original_function (lua_State *L) {
        ...     /* code 1 */
        return k(L, lua_pcallk(L, n, m, h, ctx2, k), ctx1);
      }

   Note the external, explicit call to the continuation: Lua will call the
   continuation only if needed, that is, in case of errors or resuming after
   a yield. If the called function returns normally without ever yielding,
   lua_pcallk (and lua_callk) will also return normally. (Of course, instead
   of calling the continuation in that case, you can do the equivalent work
   directly inside the original function.)

   Besides the Lua state, the continuation function has two other parameters:
   the final status of the call and the context value (ctx) that was passed
   originally to lua_pcallk. Lua does not use this context value; it only
   passes this value from the original function to the continuation function.
   For lua_pcallk, the status is the same value that would be returned by
   lua_pcallk, except that it is LUA_YIELD when being executed after a yield
   (instead of LUA_OK). For lua_yieldk and lua_callk, the status is always
   LUA_YIELD when Lua calls the continuation. (For these two functions, Lua
   will not call the continuation in case of errors, because they do not
   handle errors.) Similarly, when using lua_callk, you should call the
   continuation function with LUA_OK as the status. (For lua_yieldk, there is
   not much point in calling directly the continuation function, because
   lua_yieldk usually does not return.)

   Lua treats the continuation function as if it were the original function.
   The continuation function receives the same Lua stack from the original
   function, in the same state it would be if the callee function had
   returned. (For instance, after a lua_callk the function and its arguments
   are removed from the stack and replaced by the results from the call.) It
   also has the same upvalues. Whatever it returns is handled by Lua as if it
   were the return of the original function.

4.6 – Functions and Types

   Here we list all functions and types from the C API in alphabetical order.
   Each function has an indicator like this: [-o, +p, x]

   The first field, o, is how many elements the function pops from the stack.
   The second field, p, is how many elements the function pushes onto the
   stack. (Any function always pushes its results after popping its
   arguments.) A field in the form x|y means the function can push (or pop) x
   or y elements, depending on the situation; an interrogation mark '?' means
   that we cannot know how many elements the function pops/pushes by looking
   only at its arguments. (For instance, they may depend on what is in the
   stack.) The third field, x, tells whether the function may raise errors:
   '-' means the function never raises any error; 'm' means the function may
   raise only out-of-memory errors; 'v' means the function may raise the
   errors explained in the text; 'e' means the function can run arbitrary Lua
   code, either directly or through metamethods, and therefore may raise any
   errors.

     ----------------------------------------------------------------------

  lua_absindex

   [-0, +0, –]

 int lua_absindex (lua_State *L, int idx);

   Converts the acceptable index idx into an equivalent absolute index (that
   is, one that does not depend on the stack size).

     ----------------------------------------------------------------------

  lua_Alloc

 typedef void * (*lua_Alloc) (void *ud,
                              void *ptr,
                              size_t osize,
                              size_t nsize);

   The type of the memory-allocation function used by Lua states. The
   allocator function must provide a functionality similar to realloc, but
   not exactly the same. Its arguments are ud, an opaque pointer passed to
   lua_newstate; ptr, a pointer to the block being
   allocated/reallocated/freed; osize, the original size of the block or some
   code about what is being allocated; and nsize, the new size of the block.

   When ptr is not NULL, osize is the size of the block pointed by ptr, that
   is, the size given when it was allocated or reallocated.

   When ptr is NULL, osize encodes the kind of object that Lua is allocating.
   osize is any of LUA_TSTRING, LUA_TTABLE, LUA_TFUNCTION, LUA_TUSERDATA, or
   LUA_TTHREAD when (and only when) Lua is creating a new object of that
   type. When osize is some other value, Lua is allocating memory for
   something else.

   Lua assumes the following behavior from the allocator function:

   When nsize is zero, the allocator must behave like free and then return
   NULL.

   When nsize is not zero, the allocator must behave like realloc. In
   particular, the allocator returns NULL if and only if it cannot fulfill
   the request.

   Here is a simple implementation for the allocator function. It is used in
   the auxiliary library by luaL_newstate.

      static void *l_alloc (void *ud, void *ptr, size_t osize,
                                                 size_t nsize) {
        (void)ud;  (void)osize;  /* not used */
        if (nsize == 0) {
          free(ptr);
          return NULL;
        }
        else
          return realloc(ptr, nsize);
      }

   Note that Standard C ensures that free(NULL) has no effect and that
   realloc(NULL,size) is equivalent to malloc(size).

     ----------------------------------------------------------------------

  lua_arith

   [-(2|1), +1, e]

 void lua_arith (lua_State *L, int op);

   Performs an arithmetic or bitwise operation over the two values (or one,
   in the case of negations) at the top of the stack, with the value on the
   top being the second operand, pops these values, and pushes the result of
   the operation. The function follows the semantics of the corresponding Lua
   operator (that is, it may call metamethods).

   The value of op must be one of the following constants:
     * LUA_OPADD: performs addition (+)
     * LUA_OPSUB: performs subtraction (-)
     * LUA_OPMUL: performs multiplication (*)
     * LUA_OPDIV: performs float division (/)
     * LUA_OPIDIV: performs floor division (//)
     * LUA_OPMOD: performs modulo (%)
     * LUA_OPPOW: performs exponentiation (^)
     * LUA_OPUNM: performs mathematical negation (unary -)
     * LUA_OPBNOT: performs bitwise NOT (~)
     * LUA_OPBAND: performs bitwise AND (&)
     * LUA_OPBOR: performs bitwise OR (|)
     * LUA_OPBXOR: performs bitwise exclusive OR (~)
     * LUA_OPSHL: performs left shift (<<)
     * LUA_OPSHR: performs right shift (>>)

     ----------------------------------------------------------------------

  lua_atpanic

   [-0, +0, –]

 lua_CFunction lua_atpanic (lua_State *L, lua_CFunction panicf);

   Sets a new panic function and returns the old one (see §4.4).

     ----------------------------------------------------------------------

  lua_call

   [-(nargs+1), +nresults, e]

 void lua_call (lua_State *L, int nargs, int nresults);

   Calls a function. Like regular Lua calls, lua_call respects the __call
   metamethod. So, here the word "function" means any callable value.

   To do a call you must use the following protocol: first, the function to
   be called is pushed onto the stack; then, the arguments to the call are
   pushed in direct order; that is, the first argument is pushed first.
   Finally you call lua_call; nargs is the number of arguments that you
   pushed onto the stack. When the function returns, all arguments and the
   function value are popped and the call results are pushed onto the stack.
   The number of results is adjusted to nresults, unless nresults is
   LUA_MULTRET. In this case, all results from the function are pushed; Lua
   takes care that the returned values fit into the stack space, but it does
   not ensure any extra space in the stack. The function results are pushed
   onto the stack in direct order (the first result is pushed first), so that
   after the call the last result is on the top of the stack.

   Any error while calling and running the function is propagated upwards
   (with a longjmp).

   The following example shows how the host program can do the equivalent to
   this Lua code:

      a = f("how", t.x, 14)

   Here it is in C:

      lua_getglobal(L, "f");                  /* function to be called */
      lua_pushliteral(L, "how");                       /* 1st argument */
      lua_getglobal(L, "t");                    /* table to be indexed */
      lua_getfield(L, -1, "x");        /* push result of t.x (2nd arg) */
      lua_remove(L, -2);                  /* remove 't' from the stack */
      lua_pushinteger(L, 14);                          /* 3rd argument */
      lua_call(L, 3, 1);     /* call 'f' with 3 arguments and 1 result */
      lua_setglobal(L, "a");                         /* set global 'a' */

   Note that the code above is balanced: at its end, the stack is back to its
   original configuration. This is considered good programming practice.

     ----------------------------------------------------------------------

  lua_callk

   [-(nargs + 1), +nresults, e]

 void lua_callk (lua_State *L,
                 int nargs,
                 int nresults,
                 lua_KContext ctx,
                 lua_KFunction k);

   This function behaves exactly like lua_call, but allows the called
   function to yield (see §4.5).

     ----------------------------------------------------------------------

  lua_CFunction

 typedef int (*lua_CFunction) (lua_State *L);

   Type for C functions.

   In order to communicate properly with Lua, a C function must use the
   following protocol, which defines the way parameters and results are
   passed: a C function receives its arguments from Lua in its stack in
   direct order (the first argument is pushed first). So, when the function
   starts, lua_gettop(L) returns the number of arguments received by the
   function. The first argument (if any) is at index 1 and its last argument
   is at index lua_gettop(L). To return values to Lua, a C function just
   pushes them onto the stack, in direct order (the first result is pushed
   first), and returns in C the number of results. Any other value in the
   stack below the results will be properly discarded by Lua. Like a Lua
   function, a C function called by Lua can also return many results.

   As an example, the following function receives a variable number of
   numeric arguments and returns their average and their sum:

      static int foo (lua_State *L) {
        int n = lua_gettop(L);    /* number of arguments */
        lua_Number sum = 0.0;
        int i;
        for (i = 1; i <= n; i++) {
          if (!lua_isnumber(L, i)) {
            lua_pushliteral(L, "incorrect argument");
            lua_error(L);
          }
          sum += lua_tonumber(L, i);
        }
        lua_pushnumber(L, sum/n);        /* first result */
        lua_pushnumber(L, sum);         /* second result */
        return 2;                   /* number of results */
      }

     ----------------------------------------------------------------------

  lua_checkstack

   [-0, +0, –]

 int lua_checkstack (lua_State *L, int n);

   Ensures that the stack has space for at least n extra elements, that is,
   that you can safely push up to n values into it. It returns false if it
   cannot fulfill the request, either because it would cause the stack to be
   greater than a fixed maximum size (typically at least several thousand
   elements) or because it cannot allocate memory for the extra space. This
   function never shrinks the stack; if the stack already has space for the
   extra elements, it is left unchanged.

     ----------------------------------------------------------------------

  lua_close

   [-0, +0, –]

 void lua_close (lua_State *L);

   Close all active to-be-closed variables in the main thread, release all
   objects in the given Lua state (calling the corresponding
   garbage-collection metamethods, if any), and frees all dynamic memory used
   by this state.

   On several platforms, you may not need to call this function, because all
   resources are naturally released when the host program ends. On the other
   hand, long-running programs that create multiple states, such as daemons
   or web servers, will probably need to close states as soon as they are not
   needed.

     ----------------------------------------------------------------------

  lua_closeslot

   [-0, +0, e]

 void lua_closeslot (lua_State *L, int index);

   Close the to-be-closed slot at the given index and set its value to nil.
   The index must be the last index previously marked to be closed (see
   lua_toclose) that is still active (that is, not closed yet).

   A __close metamethod cannot yield when called through this function.

   (Exceptionally, this function was introduced in release 5.4.3. It is not
   present in previous 5.4 releases.)

     ----------------------------------------------------------------------

  lua_compare

   [-0, +0, e]

 int lua_compare (lua_State *L, int index1, int index2, int op);

   Compares two Lua values. Returns 1 if the value at index index1 satisfies
   op when compared with the value at index index2, following the semantics
   of the corresponding Lua operator (that is, it may call metamethods).
   Otherwise returns 0. Also returns 0 if any of the indices is not valid.

   The value of op must be one of the following constants:
     * LUA_OPEQ: compares for equality (==)
     * LUA_OPLT: compares for less than (<)
     * LUA_OPLE: compares for less or equal (<=)

     ----------------------------------------------------------------------

  lua_concat

   [-n, +1, e]

 void lua_concat (lua_State *L, int n);

   Concatenates the n values at the top of the stack, pops them, and leaves
   the result on the top. If n is 1, the result is the single value on the
   stack (that is, the function does nothing); if n is 0, the result is the
   empty string. Concatenation is performed following the usual semantics of
   Lua (see §3.4.6).

     ----------------------------------------------------------------------

  lua_copy

   [-0, +0, –]

 void lua_copy (lua_State *L, int fromidx, int toidx);

   Copies the element at index fromidx into the valid index toidx, replacing
   the value at that position. Values at other positions are not affected.

     ----------------------------------------------------------------------

  lua_createtable

   [-0, +1, m]

 void lua_createtable (lua_State *L, int narr, int nrec);

   Creates a new empty table and pushes it onto the stack. Parameter narr is
   a hint for how many elements the table will have as a sequence; parameter
   nrec is a hint for how many other elements the table will have. Lua may
   use these hints to preallocate memory for the new table. This
   preallocation may help performance when you know in advance how many
   elements the table will have. Otherwise you can use the function
   lua_newtable.

     ----------------------------------------------------------------------

  lua_dump

   [-0, +0, –]

 int lua_dump (lua_State *L,
                         lua_Writer writer,
                         void *data,
                         int strip);

   Dumps a function as a binary chunk. Receives a Lua function on the top of
   the stack and produces a binary chunk that, if loaded again, results in a
   function equivalent to the one dumped. As it produces parts of the chunk,
   lua_dump calls function writer (see lua_Writer) with the given data to
   write them.

   If strip is true, the binary representation may not include all debug
   information about the function, to save space.

   The value returned is the error code returned by the last call to the
   writer; 0 means no errors.

   This function does not pop the Lua function from the stack.

     ----------------------------------------------------------------------

  lua_error

   [-1, +0, v]

 int lua_error (lua_State *L);

   Raises a Lua error, using the value on the top of the stack as the error
   object. This function does a long jump, and therefore never returns (see
   luaL_error).

     ----------------------------------------------------------------------

  lua_gc

   [-0, +0, –]

 int lua_gc (lua_State *L, int what, ...);

   Controls the garbage collector.

   This function performs several tasks, according to the value of the
   parameter what. For options that need extra arguments, they are listed
   after the option.
     * LUA_GCCOLLECT: Performs a full garbage-collection cycle.
     * LUA_GCSTOP: Stops the garbage collector.
     * LUA_GCRESTART: Restarts the garbage collector.
     * LUA_GCCOUNT: Returns the current amount of memory (in Kbytes) in use
       by Lua.
     * LUA_GCCOUNTB: Returns the remainder of dividing the current amount of
       bytes of memory in use by Lua by 1024.
     * LUA_GCSTEP (int stepsize): Performs an incremental step of garbage
       collection, corresponding to the allocation of stepsize Kbytes.
     * LUA_GCISRUNNING: Returns a boolean that tells whether the collector is
       running (i.e., not stopped).
     * LUA_GCINC (int pause, int stepmul, stepsize): Changes the collector to
       incremental mode with the given parameters (see §2.5.1). Returns the
       previous mode (LUA_GCGEN or LUA_GCINC).
     * LUA_GCGEN (int minormul, int majormul): Changes the collector to
       generational mode with the given parameters (see §2.5.2). Returns the
       previous mode (LUA_GCGEN or LUA_GCINC).

   For more details about these options, see collectgarbage.

   This function should not be called by a finalizer.

     ----------------------------------------------------------------------

  lua_getallocf

   [-0, +0, –]

 lua_Alloc lua_getallocf (lua_State *L, void **ud);

   Returns the memory-allocation function of a given state. If ud is not
   NULL, Lua stores in *ud the opaque pointer given when the memory-allocator
   function was set.

     ----------------------------------------------------------------------

  lua_getfield

   [-0, +1, e]

 int lua_getfield (lua_State *L, int index, const char *k);

   Pushes onto the stack the value t[k], where t is the value at the given
   index. As in Lua, this function may trigger a metamethod for the "index"
   event (see §2.4).

   Returns the type of the pushed value.

     ----------------------------------------------------------------------

  lua_getextraspace

   [-0, +0, –]

 void *lua_getextraspace (lua_State *L);

   Returns a pointer to a raw memory area associated with the given Lua
   state. The application can use this area for any purpose; Lua does not use
   it for anything.

   Each new thread has this area initialized with a copy of the area of the
   main thread.

   By default, this area has the size of a pointer to void, but you can
   recompile Lua with a different size for this area. (See LUA_EXTRASPACE in
   luaconf.h.)

     ----------------------------------------------------------------------

  lua_getglobal

   [-0, +1, e]

 int lua_getglobal (lua_State *L, const char *name);

   Pushes onto the stack the value of the global name. Returns the type of
   that value.

     ----------------------------------------------------------------------

  lua_geti

   [-0, +1, e]

 int lua_geti (lua_State *L, int index, lua_Integer i);

   Pushes onto the stack the value t[i], where t is the value at the given
   index. As in Lua, this function may trigger a metamethod for the "index"
   event (see §2.4).

   Returns the type of the pushed value.

     ----------------------------------------------------------------------

  lua_getmetatable

   [-0, +(0|1), –]

 int lua_getmetatable (lua_State *L, int index);

   If the value at the given index has a metatable, the function pushes that
   metatable onto the stack and returns 1. Otherwise, the function returns 0
   and pushes nothing on the stack.

     ----------------------------------------------------------------------

  lua_gettable

   [-1, +1, e]

 int lua_gettable (lua_State *L, int index);

   Pushes onto the stack the value t[k], where t is the value at the given
   index and k is the value on the top of the stack.

   This function pops the key from the stack, pushing the resulting value in
   its place. As in Lua, this function may trigger a metamethod for the
   "index" event (see §2.4).

   Returns the type of the pushed value.

     ----------------------------------------------------------------------

  lua_gettop

   [-0, +0, –]

 int lua_gettop (lua_State *L);

   Returns the index of the top element in the stack. Because indices start
   at 1, this result is equal to the number of elements in the stack; in
   particular, 0 means an empty stack.

     ----------------------------------------------------------------------

  lua_getiuservalue

   [-0, +1, –]

 int lua_getiuservalue (lua_State *L, int index, int n);

   Pushes onto the stack the n-th user value associated with the full
   userdata at the given index and returns the type of the pushed value.

   If the userdata does not have that value, pushes nil and returns
   LUA_TNONE.

     ----------------------------------------------------------------------

  lua_insert

   [-1, +1, –]

 void lua_insert (lua_State *L, int index);

   Moves the top element into the given valid index, shifting up the elements
   above this index to open space. This function cannot be called with a
   pseudo-index, because a pseudo-index is not an actual stack position.

     ----------------------------------------------------------------------

  lua_Integer

 typedef ... lua_Integer;

   The type of integers in Lua.

   By default this type is long long, (usually a 64-bit two-complement
   integer), but that can be changed to long or int (usually a 32-bit
   two-complement integer). (See LUA_INT_TYPE in luaconf.h.)

   Lua also defines the constants LUA_MININTEGER and LUA_MAXINTEGER, with the
   minimum and the maximum values that fit in this type.

     ----------------------------------------------------------------------

  lua_isboolean

   [-0, +0, –]

 int lua_isboolean (lua_State *L, int index);

   Returns 1 if the value at the given index is a boolean, and 0 otherwise.

     ----------------------------------------------------------------------

  lua_iscfunction

   [-0, +0, –]

 int lua_iscfunction (lua_State *L, int index);

   Returns 1 if the value at the given index is a C function, and
   0 otherwise.

     ----------------------------------------------------------------------

  lua_isfunction

   [-0, +0, –]

 int lua_isfunction (lua_State *L, int index);

   Returns 1 if the value at the given index is a function (either C or Lua),
   and 0 otherwise.

     ----------------------------------------------------------------------

  lua_isinteger

   [-0, +0, –]

 int lua_isinteger (lua_State *L, int index);

   Returns 1 if the value at the given index is an integer (that is, the
   value is a number and is represented as an integer), and 0 otherwise.

     ----------------------------------------------------------------------

  lua_islightuserdata

   [-0, +0, –]

 int lua_islightuserdata (lua_State *L, int index);

   Returns 1 if the value at the given index is a light userdata, and
   0 otherwise.

     ----------------------------------------------------------------------

  lua_isnil

   [-0, +0, –]

 int lua_isnil (lua_State *L, int index);

   Returns 1 if the value at the given index is nil, and 0 otherwise.

     ----------------------------------------------------------------------

  lua_isnone

   [-0, +0, –]

 int lua_isnone (lua_State *L, int index);

   Returns 1 if the given index is not valid, and 0 otherwise.

     ----------------------------------------------------------------------

  lua_isnoneornil

   [-0, +0, –]

 int lua_isnoneornil (lua_State *L, int index);

   Returns 1 if the given index is not valid or if the value at this index is
   nil, and 0 otherwise.

     ----------------------------------------------------------------------

  lua_isnumber

   [-0, +0, –]

 int lua_isnumber (lua_State *L, int index);

   Returns 1 if the value at the given index is a number or a string
   convertible to a number, and 0 otherwise.

     ----------------------------------------------------------------------

  lua_isstring

   [-0, +0, –]

 int lua_isstring (lua_State *L, int index);

   Returns 1 if the value at the given index is a string or a number (which
   is always convertible to a string), and 0 otherwise.

     ----------------------------------------------------------------------

  lua_istable

   [-0, +0, –]

 int lua_istable (lua_State *L, int index);

   Returns 1 if the value at the given index is a table, and 0 otherwise.

     ----------------------------------------------------------------------

  lua_isthread

   [-0, +0, –]

 int lua_isthread (lua_State *L, int index);

   Returns 1 if the value at the given index is a thread, and 0 otherwise.

     ----------------------------------------------------------------------

  lua_isuserdata

   [-0, +0, –]

 int lua_isuserdata (lua_State *L, int index);

   Returns 1 if the value at the given index is a userdata (either full or
   light), and 0 otherwise.

     ----------------------------------------------------------------------

  lua_isyieldable

   [-0, +0, –]

 int lua_isyieldable (lua_State *L);

   Returns 1 if the given coroutine can yield, and 0 otherwise.

     ----------------------------------------------------------------------

  lua_KContext

 typedef ... lua_KContext;

   The type for continuation-function contexts. It must be a numeric type.
   This type is defined as intptr_t when intptr_t is available, so that it
   can store pointers too. Otherwise, it is defined as ptrdiff_t.

     ----------------------------------------------------------------------

  lua_KFunction

 typedef int (*lua_KFunction) (lua_State *L, int status, lua_KContext ctx);

   Type for continuation functions (see §4.5).

     ----------------------------------------------------------------------

  lua_len

   [-0, +1, e]

 void lua_len (lua_State *L, int index);

   Returns the length of the value at the given index. It is equivalent to
   the '#' operator in Lua (see §3.4.7) and may trigger a metamethod for the
   "length" event (see §2.4). The result is pushed on the stack.

     ----------------------------------------------------------------------

  lua_load

   [-0, +1, –]

 int lua_load (lua_State *L,
               lua_Reader reader,
               void *data,
               const char *chunkname,
               const char *mode);

   Loads a Lua chunk without running it. If there are no errors, lua_load
   pushes the compiled chunk as a Lua function on top of the stack.
   Otherwise, it pushes an error message.

   The lua_load function uses a user-supplied reader function to read the
   chunk (see lua_Reader). The data argument is an opaque value passed to the
   reader function.

   The chunkname argument gives a name to the chunk, which is used for error
   messages and in debug information (see §4.7).

   lua_load automatically detects whether the chunk is text or binary and
   loads it accordingly (see program luac). The string mode works as in
   function load, with the addition that a NULL value is equivalent to the
   string "bt".

   lua_load uses the stack internally, so the reader function must always
   leave the stack unmodified when returning.

   lua_load can return LUA_OK, LUA_ERRSYNTAX, or LUA_ERRMEM. The function may
   also return other values corresponding to errors raised by the read
   function (see §4.4.1).

   If the resulting function has upvalues, its first upvalue is set to the
   value of the global environment stored at index LUA_RIDX_GLOBALS in the
   registry (see §4.3). When loading main chunks, this upvalue will be the
   _ENV variable (see §2.2). Other upvalues are initialized with nil.

     ----------------------------------------------------------------------

  lua_newstate

   [-0, +0, –]

 lua_State *lua_newstate (lua_Alloc f, void *ud);

   Creates a new independent state and returns its main thread. Returns NULL
   if it cannot create the state (due to lack of memory). The argument f is
   the allocator function; Lua will do all memory allocation for this state
   through this function (see lua_Alloc). The second argument, ud, is an
   opaque pointer that Lua passes to the allocator in every call.

     ----------------------------------------------------------------------

  lua_newtable

   [-0, +1, m]

 void lua_newtable (lua_State *L);

   Creates a new empty table and pushes it onto the stack. It is equivalent
   to lua_createtable(L, 0, 0).

     ----------------------------------------------------------------------

  lua_newthread

   [-0, +1, m]

 lua_State *lua_newthread (lua_State *L);

   Creates a new thread, pushes it on the stack, and returns a pointer to a
   lua_State that represents this new thread. The new thread returned by this
   function shares with the original thread its global environment, but has
   an independent execution stack.

   Threads are subject to garbage collection, like any Lua object.

     ----------------------------------------------------------------------

  lua_newuserdatauv

   [-0, +1, m]

 void *lua_newuserdatauv (lua_State *L, size_t size, int nuvalue);

   This function creates and pushes on the stack a new full userdata, with
   nuvalue associated Lua values, called user values, plus an associated
   block of raw memory with size bytes. (The user values can be set and read
   with the functions lua_setiuservalue and lua_getiuservalue.)

   The function returns the address of the block of memory. Lua ensures that
   this address is valid as long as the corresponding userdata is alive (see
   §2.5). Moreover, if the userdata is marked for finalization (see §2.5.3),
   its address is valid at least until the call to its finalizer.

     ----------------------------------------------------------------------

  lua_next

   [-1, +(2|0), v]

 int lua_next (lua_State *L, int index);

   Pops a key from the stack, and pushes a key–value pair from the table at
   the given index, the "next" pair after the given key. If there are no more
   elements in the table, then lua_next returns 0 and pushes nothing.

   A typical table traversal looks like this:

      /* table is in the stack at index 't' */
      lua_pushnil(L);  /* first key */
      while (lua_next(L, t) != 0) {
        /* uses 'key' (at index -2) and 'value' (at index -1) */
        printf("%s - %s\n",
               lua_typename(L, lua_type(L, -2)),
               lua_typename(L, lua_type(L, -1)));
        /* removes 'value'; keeps 'key' for next iteration */
        lua_pop(L, 1);
      }

   While traversing a table, avoid calling lua_tolstring directly on a key,
   unless you know that the key is actually a string. Recall that
   lua_tolstring may change the value at the given index; this confuses the
   next call to lua_next.

   This function may raise an error if the given key is neither nil nor
   present in the table. See function next for the caveats of modifying the
   table during its traversal.

     ----------------------------------------------------------------------

  lua_Number

 typedef ... lua_Number;

   The type of floats in Lua.

   By default this type is double, but that can be changed to a single float
   or a long double. (See LUA_FLOAT_TYPE in luaconf.h.)

     ----------------------------------------------------------------------

  lua_numbertointeger

 int lua_numbertointeger (lua_Number n, lua_Integer *p);

   Tries to convert a Lua float to a Lua integer; the float n must have an
   integral value. If that value is within the range of Lua integers, it is
   converted to an integer and assigned to *p. The macro results in a boolean
   indicating whether the conversion was successful. (Note that this range
   test can be tricky to do correctly without this macro, due to rounding.)

   This macro may evaluate its arguments more than once.

     ----------------------------------------------------------------------

  lua_pcall

   [-(nargs + 1), +(nresults|1), –]

 int lua_pcall (lua_State *L, int nargs, int nresults, int msgh);

   Calls a function (or a callable object) in protected mode.

   Both nargs and nresults have the same meaning as in lua_call. If there are
   no errors during the call, lua_pcall behaves exactly like lua_call.
   However, if there is any error, lua_pcall catches it, pushes a single
   value on the stack (the error object), and returns an error code. Like
   lua_call, lua_pcall always removes the function and its arguments from the
   stack.

   If msgh is 0, then the error object returned on the stack is exactly the
   original error object. Otherwise, msgh is the stack index of a message
   handler. (This index cannot be a pseudo-index.) In case of runtime errors,
   this handler will be called with the error object and its return value
   will be the object returned on the stack by lua_pcall.

   Typically, the message handler is used to add more debug information to
   the error object, such as a stack traceback. Such information cannot be
   gathered after the return of lua_pcall, since by then the stack has
   unwound.

   The lua_pcall function returns one of the following status codes: LUA_OK,
   LUA_ERRRUN, LUA_ERRMEM, or LUA_ERRERR.

     ----------------------------------------------------------------------

  lua_pcallk

   [-(nargs + 1), +(nresults|1), –]

 int lua_pcallk (lua_State *L,
                 int nargs,
                 int nresults,
                 int msgh,
                 lua_KContext ctx,
                 lua_KFunction k);

   This function behaves exactly like lua_pcall, except that it allows the
   called function to yield (see §4.5).

     ----------------------------------------------------------------------

  lua_pop

   [-n, +0, e]

 void lua_pop (lua_State *L, int n);

   Pops n elements from the stack. It is implemented as a macro over
   lua_settop.

     ----------------------------------------------------------------------

  lua_pushboolean

   [-0, +1, –]

 void lua_pushboolean (lua_State *L, int b);

   Pushes a boolean value with value b onto the stack.

     ----------------------------------------------------------------------

  lua_pushcclosure

   [-n, +1, m]

 void lua_pushcclosure (lua_State *L, lua_CFunction fn, int n);

   Pushes a new C closure onto the stack. This function receives a pointer to
   a C function and pushes onto the stack a Lua value of type function that,
   when called, invokes the corresponding C function. The parameter n tells
   how many upvalues this function will have (see §4.2).

   Any function to be callable by Lua must follow the correct protocol to
   receive its parameters and return its results (see lua_CFunction).

   When a C function is created, it is possible to associate some values with
   it, the so called upvalues; these upvalues are then accessible to the
   function whenever it is called. This association is called a C closure
   (see §4.2). To create a C closure, first the initial values for its
   upvalues must be pushed onto the stack. (When there are multiple upvalues,
   the first value is pushed first.) Then lua_pushcclosure is called to
   create and push the C function onto the stack, with the argument n telling
   how many values will be associated with the function. lua_pushcclosure
   also pops these values from the stack.

   The maximum value for n is 255.

   When n is zero, this function creates a light C function, which is just a
   pointer to the C function. In that case, it never raises a memory error.

     ----------------------------------------------------------------------

  lua_pushcfunction

   [-0, +1, –]

 void lua_pushcfunction (lua_State *L, lua_CFunction f);

   Pushes a C function onto the stack. This function is equivalent to
   lua_pushcclosure with no upvalues.

     ----------------------------------------------------------------------

  lua_pushfstring

   [-0, +1, v]

 const char *lua_pushfstring (lua_State *L, const char *fmt, ...);

   Pushes onto the stack a formatted string and returns a pointer to this
   string (see §4.1.3). It is similar to the ISO C function sprintf, but has
   two important differences. First, you do not have to allocate space for
   the result; the result is a Lua string and Lua takes care of memory
   allocation (and deallocation, through garbage collection). Second, the
   conversion specifiers are quite restricted. There are no flags, widths, or
   precisions. The conversion specifiers can only be '%%' (inserts the
   character '%'), '%s' (inserts a zero-terminated string, with no size
   restrictions), '%f' (inserts a lua_Number), '%I' (inserts a lua_Integer),
   '%p' (inserts a pointer), '%d' (inserts an int), '%c' (inserts an int as a
   one-byte character), and '%U' (inserts a long int as a UTF-8 byte
   sequence).

   This function may raise errors due to memory overflow or an invalid
   conversion specifier.

     ----------------------------------------------------------------------

  lua_pushglobaltable

   [-0, +1, –]

 void lua_pushglobaltable (lua_State *L);

   Pushes the global environment onto the stack.

     ----------------------------------------------------------------------

  lua_pushinteger

   [-0, +1, –]

 void lua_pushinteger (lua_State *L, lua_Integer n);

   Pushes an integer with value n onto the stack.

     ----------------------------------------------------------------------

  lua_pushlightuserdata

   [-0, +1, –]

 void lua_pushlightuserdata (lua_State *L, void *p);

   Pushes a light userdata onto the stack.

   Userdata represent C values in Lua. A light userdata represents a pointer,
   a void*. It is a value (like a number): you do not create it, it has no
   individual metatable, and it is not collected (as it was never created). A
   light userdata is equal to "any" light userdata with the same C address.

     ----------------------------------------------------------------------

  lua_pushliteral

   [-0, +1, m]

 const char *lua_pushliteral (lua_State *L, const char *s);

   This macro is equivalent to lua_pushstring, but should be used only when s
   is a literal string. (Lua may optimize this case.)

     ----------------------------------------------------------------------

  lua_pushlstring

   [-0, +1, m]

 const char *lua_pushlstring (lua_State *L, const char *s, size_t len);

   Pushes the string pointed to by s with size len onto the stack. Lua will
   make or reuse an internal copy of the given string, so the memory at s can
   be freed or reused immediately after the function returns. The string can
   contain any binary data, including embedded zeros.

   Returns a pointer to the internal copy of the string (see §4.1.3).

     ----------------------------------------------------------------------

  lua_pushnil

   [-0, +1, –]

 void lua_pushnil (lua_State *L);

   Pushes a nil value onto the stack.

     ----------------------------------------------------------------------

  lua_pushnumber

   [-0, +1, –]

 void lua_pushnumber (lua_State *L, lua_Number n);

   Pushes a float with value n onto the stack.

     ----------------------------------------------------------------------

  lua_pushstring

   [-0, +1, m]

 const char *lua_pushstring (lua_State *L, const char *s);

   Pushes the zero-terminated string pointed to by s onto the stack. Lua will
   make or reuse an internal copy of the given string, so the memory at s can
   be freed or reused immediately after the function returns.

   Returns a pointer to the internal copy of the string (see §4.1.3).

   If s is NULL, pushes nil and returns NULL.

     ----------------------------------------------------------------------

  lua_pushthread

   [-0, +1, –]

 int lua_pushthread (lua_State *L);

   Pushes the thread represented by L onto the stack. Returns 1 if this
   thread is the main thread of its state.

     ----------------------------------------------------------------------

  lua_pushvalue

   [-0, +1, –]

 void lua_pushvalue (lua_State *L, int index);

   Pushes a copy of the element at the given index onto the stack.

     ----------------------------------------------------------------------

  lua_pushvfstring

   [-0, +1, v]

 const char *lua_pushvfstring (lua_State *L,
                               const char *fmt,
                               va_list argp);

   Equivalent to lua_pushfstring, except that it receives a va_list instead
   of a variable number of arguments.

     ----------------------------------------------------------------------

  lua_rawequal

   [-0, +0, –]

 int lua_rawequal (lua_State *L, int index1, int index2);

   Returns 1 if the two values in indices index1 and index2 are primitively
   equal (that is, equal without calling the __eq metamethod). Otherwise
   returns 0. Also returns 0 if any of the indices are not valid.

     ----------------------------------------------------------------------

  lua_rawget

   [-1, +1, –]

 int lua_rawget (lua_State *L, int index);

   Similar to lua_gettable, but does a raw access (i.e., without
   metamethods).

     ----------------------------------------------------------------------

  lua_rawgeti

   [-0, +1, –]

 int lua_rawgeti (lua_State *L, int index, lua_Integer n);

   Pushes onto the stack the value t[n], where t is the table at the given
   index. The access is raw, that is, it does not use the __index metavalue.

   Returns the type of the pushed value.

     ----------------------------------------------------------------------

  lua_rawgetp

   [-0, +1, –]

 int lua_rawgetp (lua_State *L, int index, const void *p);

   Pushes onto the stack the value t[k], where t is the table at the given
   index and k is the pointer p represented as a light userdata. The access
   is raw; that is, it does not use the __index metavalue.

   Returns the type of the pushed value.

     ----------------------------------------------------------------------

  lua_rawlen

   [-0, +0, –]

 lua_Unsigned lua_rawlen (lua_State *L, int index);

   Returns the raw "length" of the value at the given index: for strings,
   this is the string length; for tables, this is the result of the length
   operator ('#') with no metamethods; for userdata, this is the size of the
   block of memory allocated for the userdata. For other values, this call
   returns 0.

     ----------------------------------------------------------------------

  lua_rawset

   [-2, +0, m]

 void lua_rawset (lua_State *L, int index);

   Similar to lua_settable, but does a raw assignment (i.e., without
   metamethods).

     ----------------------------------------------------------------------

  lua_rawseti

   [-1, +0, m]

 void lua_rawseti (lua_State *L, int index, lua_Integer i);

   Does the equivalent of t[i] = v, where t is the table at the given index
   and v is the value on the top of the stack.

   This function pops the value from the stack. The assignment is raw, that
   is, it does not use the __newindex metavalue.

     ----------------------------------------------------------------------

  lua_rawsetp

   [-1, +0, m]

 void lua_rawsetp (lua_State *L, int index, const void *p);

   Does the equivalent of t[p] = v, where t is the table at the given index,
   p is encoded as a light userdata, and v is the value on the top of the
   stack.

   This function pops the value from the stack. The assignment is raw, that
   is, it does not use the __newindex metavalue.

     ----------------------------------------------------------------------

  lua_Reader

 typedef const char * (*lua_Reader) (lua_State *L,
                                     void *data,
                                     size_t *size);

   The reader function used by lua_load. Every time lua_load needs another
   piece of the chunk, it calls the reader, passing along its data parameter.
   The reader must return a pointer to a block of memory with a new piece of
   the chunk and set size to the block size. The block must exist until the
   reader function is called again. To signal the end of the chunk, the
   reader must return NULL or set size to zero. The reader function may
   return pieces of any size greater than zero.

     ----------------------------------------------------------------------

  lua_register

   [-0, +0, e]

 void lua_register (lua_State *L, const char *name, lua_CFunction f);

   Sets the C function f as the new value of global name. It is defined as a
   macro:

      #define lua_register(L,n,f) \
             (lua_pushcfunction(L, f), lua_setglobal(L, n))

     ----------------------------------------------------------------------

  lua_remove

   [-1, +0, –]

 void lua_remove (lua_State *L, int index);

   Removes the element at the given valid index, shifting down the elements
   above this index to fill the gap. This function cannot be called with a
   pseudo-index, because a pseudo-index is not an actual stack position.

     ----------------------------------------------------------------------

  lua_replace

   [-1, +0, –]

 void lua_replace (lua_State *L, int index);

   Moves the top element into the given valid index without shifting any
   element (therefore replacing the value at that given index), and then pops
   the top element.

     ----------------------------------------------------------------------

  lua_resetthread

   [-0, +?, –]

 int lua_resetthread (lua_State *L);

   Resets a thread, cleaning its call stack and closing all pending
   to-be-closed variables. Returns a status code: LUA_OK for no errors in the
   thread (either the original error that stopped the thread or errors in
   closing methods), or an error status otherwise. In case of error, leaves
   the error object on the top of the stack.

     ----------------------------------------------------------------------

  lua_resume

   [-?, +?, –]

 int lua_resume (lua_State *L, lua_State *from, int nargs,
                           int *nresults);

   Starts and resumes a coroutine in the given thread L.

   To start a coroutine, you push the main function plus any arguments onto
   the empty stack of the thread. then you call lua_resume, with nargs being
   the number of arguments. This call returns when the coroutine suspends or
   finishes its execution. When it returns, *nresults is updated and the top
   of the stack contains the *nresults values passed to lua_yield or returned
   by the body function. lua_resume returns LUA_YIELD if the coroutine
   yields, LUA_OK if the coroutine finishes its execution without errors, or
   an error code in case of errors (see §4.4.1). In case of errors, the error
   object is on the top of the stack.

   To resume a coroutine, you remove the *nresults yielded values from its
   stack, push the values to be passed as results from yield, and then call
   lua_resume.

   The parameter from represents the coroutine that is resuming L. If there
   is no such coroutine, this parameter can be NULL.

     ----------------------------------------------------------------------

  lua_rotate

   [-0, +0, –]

 void lua_rotate (lua_State *L, int idx, int n);

   Rotates the stack elements between the valid index idx and the top of the
   stack. The elements are rotated n positions in the direction of the top,
   for a positive n, or -n positions in the direction of the bottom, for a
   negative n. The absolute value of n must not be greater than the size of
   the slice being rotated. This function cannot be called with a
   pseudo-index, because a pseudo-index is not an actual stack position.

     ----------------------------------------------------------------------

  lua_setallocf

   [-0, +0, –]

 void lua_setallocf (lua_State *L, lua_Alloc f, void *ud);

   Changes the allocator function of a given state to f with user data ud.

     ----------------------------------------------------------------------

  lua_setfield

   [-1, +0, e]

 void lua_setfield (lua_State *L, int index, const char *k);

   Does the equivalent to t[k] = v, where t is the value at the given index
   and v is the value on the top of the stack.

   This function pops the value from the stack. As in Lua, this function may
   trigger a metamethod for the "newindex" event (see §2.4).

     ----------------------------------------------------------------------

  lua_setglobal

   [-1, +0, e]

 void lua_setglobal (lua_State *L, const char *name);

   Pops a value from the stack and sets it as the new value of global name.

     ----------------------------------------------------------------------

  lua_seti

   [-1, +0, e]

 void lua_seti (lua_State *L, int index, lua_Integer n);

   Does the equivalent to t[n] = v, where t is the value at the given index
   and v is the value on the top of the stack.

   This function pops the value from the stack. As in Lua, this function may
   trigger a metamethod for the "newindex" event (see §2.4).

     ----------------------------------------------------------------------

  lua_setiuservalue

   [-1, +0, –]

 int lua_setiuservalue (lua_State *L, int index, int n);

   Pops a value from the stack and sets it as the new n-th user value
   associated to the full userdata at the given index. Returns 0 if the
   userdata does not have that value.

     ----------------------------------------------------------------------

  lua_setmetatable

   [-1, +0, –]

 int lua_setmetatable (lua_State *L, int index);

   Pops a table or nil from the stack and sets that value as the new
   metatable for the value at the given index. (nil means no metatable.)

   (For historical reasons, this function returns an int, which now is always
   1.)

     ----------------------------------------------------------------------

  lua_settable

   [-2, +0, e]

 void lua_settable (lua_State *L, int index);

   Does the equivalent to t[k] = v, where t is the value at the given index,
   v is the value on the top of the stack, and k is the value just below the
   top.

   This function pops both the key and the value from the stack. As in Lua,
   this function may trigger a metamethod for the "newindex" event (see
   §2.4).

     ----------------------------------------------------------------------

  lua_settop

   [-?, +?, e]

 void lua_settop (lua_State *L, int index);

   Accepts any index, or 0, and sets the stack top to this index. If the new
   top is greater than the old one, then the new elements are filled with
   nil. If index is 0, then all stack elements are removed.

   This function can run arbitrary code when removing an index marked as
   to-be-closed from the stack.

     ----------------------------------------------------------------------

  lua_setwarnf

   [-0, +0, –]

 void lua_setwarnf (lua_State *L, lua_WarnFunction f, void *ud);

   Sets the warning function to be used by Lua to emit warnings (see
   lua_WarnFunction). The ud parameter sets the value ud passed to the
   warning function.

     ----------------------------------------------------------------------

  lua_State

 typedef struct lua_State lua_State;

   An opaque structure that points to a thread and indirectly (through the
   thread) to the whole state of a Lua interpreter. The Lua library is fully
   reentrant: it has no global variables. All information about a state is
   accessible through this structure.

   A pointer to this structure must be passed as the first argument to every
   function in the library, except to lua_newstate, which creates a Lua state
   from scratch.

     ----------------------------------------------------------------------

  lua_status

   [-0, +0, –]

 int lua_status (lua_State *L);

   Returns the status of the thread L.

   The status can be LUA_OK for a normal thread, an error code if the thread
   finished the execution of a lua_resume with an error, or LUA_YIELD if the
   thread is suspended.

   You can call functions only in threads with status LUA_OK. You can resume
   threads with status LUA_OK (to start a new coroutine) or LUA_YIELD (to
   resume a coroutine).

     ----------------------------------------------------------------------

  lua_stringtonumber

   [-0, +1, –]

 size_t lua_stringtonumber (lua_State *L, const char *s);

   Converts the zero-terminated string s to a number, pushes that number into
   the stack, and returns the total size of the string, that is, its length
   plus one. The conversion can result in an integer or a float, according to
   the lexical conventions of Lua (see §3.1). The string may have leading and
   trailing whitespaces and a sign. If the string is not a valid numeral,
   returns 0 and pushes nothing. (Note that the result can be used as a
   boolean, true if the conversion succeeds.)

     ----------------------------------------------------------------------

  lua_toboolean

   [-0, +0, –]

 int lua_toboolean (lua_State *L, int index);

   Converts the Lua value at the given index to a C boolean value (0 or 1).
   Like all tests in Lua, lua_toboolean returns true for any Lua value
   different from false and nil; otherwise it returns false. (If you want to
   accept only actual boolean values, use lua_isboolean to test the value's
   type.)

     ----------------------------------------------------------------------

  lua_tocfunction

   [-0, +0, –]

 lua_CFunction lua_tocfunction (lua_State *L, int index);

   Converts a value at the given index to a C function. That value must be a
   C function; otherwise, returns NULL.

     ----------------------------------------------------------------------

  lua_toclose

   [-0, +0, m]

 void lua_toclose (lua_State *L, int index);

   Marks the given index in the stack as a to-be-closed slot (see §3.3.8).
   Like a to-be-closed variable in Lua, the value at that slot in the stack
   will be closed when it goes out of scope. Here, in the context of a C
   function, to go out of scope means that the running function returns to
   Lua, or there is an error, or the slot is removed from the stack through
   lua_settop or lua_pop, or there is a call to lua_closeslot. A slot marked
   as to-be-closed should not be removed from the stack by any other function
   in the API except lua_settop or lua_pop, unless previously deactivated by
   lua_closeslot.

   This function should not be called for an index that is equal to or below
   an active to-be-closed slot.

   Note that, both in case of errors and of a regular return, by the time the
   __close metamethod runs, the C stack was already unwound, so that any
   automatic C variable declared in the calling function (e.g., a buffer)
   will be out of scope.

     ----------------------------------------------------------------------

  lua_tointeger

   [-0, +0, –]

 lua_Integer lua_tointeger (lua_State *L, int index);

   Equivalent to lua_tointegerx with isnum equal to NULL.

     ----------------------------------------------------------------------

  lua_tointegerx

   [-0, +0, –]

 lua_Integer lua_tointegerx (lua_State *L, int index, int *isnum);

   Converts the Lua value at the given index to the signed integral type
   lua_Integer. The Lua value must be an integer, or a number or string
   convertible to an integer (see §3.4.3); otherwise, lua_tointegerx
   returns 0.

   If isnum is not NULL, its referent is assigned a boolean value that
   indicates whether the operation succeeded.

     ----------------------------------------------------------------------

  lua_tolstring

   [-0, +0, m]

 const char *lua_tolstring (lua_State *L, int index, size_t *len);

   Converts the Lua value at the given index to a C string. If len is not
   NULL, it sets *len with the string length. The Lua value must be a string
   or a number; otherwise, the function returns NULL. If the value is a
   number, then lua_tolstring also changes the actual value in the stack to a
   string. (This change confuses lua_next when lua_tolstring is applied to
   keys during a table traversal.)

   lua_tolstring returns a pointer to a string inside the Lua state (see
   §4.1.3). This string always has a zero ('\0') after its last character (as
   in C), but can contain other zeros in its body.

     ----------------------------------------------------------------------

  lua_tonumber

   [-0, +0, –]

 lua_Number lua_tonumber (lua_State *L, int index);

   Equivalent to lua_tonumberx with isnum equal to NULL.

     ----------------------------------------------------------------------

  lua_tonumberx

   [-0, +0, –]

 lua_Number lua_tonumberx (lua_State *L, int index, int *isnum);

   Converts the Lua value at the given index to the C type lua_Number (see
   lua_Number). The Lua value must be a number or a string convertible to a
   number (see §3.4.3); otherwise, lua_tonumberx returns 0.

   If isnum is not NULL, its referent is assigned a boolean value that
   indicates whether the operation succeeded.

     ----------------------------------------------------------------------

  lua_topointer

   [-0, +0, –]

 const void *lua_topointer (lua_State *L, int index);

   Converts the value at the given index to a generic C pointer (void*). The
   value can be a userdata, a table, a thread, a string, or a function;
   otherwise, lua_topointer returns NULL. Different objects will give
   different pointers. There is no way to convert the pointer back to its
   original value.

   Typically this function is used only for hashing and debug information.

     ----------------------------------------------------------------------

  lua_tostring

   [-0, +0, m]

 const char *lua_tostring (lua_State *L, int index);

   Equivalent to lua_tolstring with len equal to NULL.

     ----------------------------------------------------------------------

  lua_tothread

   [-0, +0, –]

 lua_State *lua_tothread (lua_State *L, int index);

   Converts the value at the given index to a Lua thread (represented as
   lua_State*). This value must be a thread; otherwise, the function returns
   NULL.

     ----------------------------------------------------------------------

  lua_touserdata

   [-0, +0, –]

 void *lua_touserdata (lua_State *L, int index);

   If the value at the given index is a full userdata, returns its
   memory-block address. If the value is a light userdata, returns its value
   (a pointer). Otherwise, returns NULL.

     ----------------------------------------------------------------------

  lua_type

   [-0, +0, –]

 int lua_type (lua_State *L, int index);

   Returns the type of the value in the given valid index, or LUA_TNONE for a
   non-valid but acceptable index. The types returned by lua_type are coded
   by the following constants defined in lua.h: LUA_TNIL, LUA_TNUMBER,
   LUA_TBOOLEAN, LUA_TSTRING, LUA_TTABLE, LUA_TFUNCTION, LUA_TUSERDATA,
   LUA_TTHREAD, and LUA_TLIGHTUSERDATA.

     ----------------------------------------------------------------------

  lua_typename

   [-0, +0, –]

 const char *lua_typename (lua_State *L, int tp);

   Returns the name of the type encoded by the value tp, which must be one
   the values returned by lua_type.

     ----------------------------------------------------------------------

  lua_Unsigned

 typedef ... lua_Unsigned;

   The unsigned version of lua_Integer.

     ----------------------------------------------------------------------

  lua_upvalueindex

   [-0, +0, –]

 int lua_upvalueindex (int i);

   Returns the pseudo-index that represents the i-th upvalue of the running
   function (see §4.2). i must be in the range [1,256].

     ----------------------------------------------------------------------

  lua_version

   [-0, +0, –]

 lua_Number lua_version (lua_State *L);

   Returns the version number of this core.

     ----------------------------------------------------------------------

  lua_WarnFunction

 typedef void (*lua_WarnFunction) (void *ud, const char *msg, int tocont);

   The type of warning functions, called by Lua to emit warnings. The first
   parameter is an opaque pointer set by lua_setwarnf. The second parameter
   is the warning message. The third parameter is a boolean that indicates
   whether the message is to be continued by the message in the next call.

   See warn for more details about warnings.

     ----------------------------------------------------------------------

  lua_warning

   [-0, +0, –]

 void lua_warning (lua_State *L, const char *msg, int tocont);

   Emits a warning with the given message. A message in a call with tocont
   true should be continued in another call to this function.

   See warn for more details about warnings.

     ----------------------------------------------------------------------

  lua_Writer

 typedef int (*lua_Writer) (lua_State *L,
                            const void* p,
                            size_t sz,
                            void* ud);

   The type of the writer function used by lua_dump. Every time lua_dump
   produces another piece of chunk, it calls the writer, passing along the
   buffer to be written (p), its size (sz), and the ud parameter supplied to
   lua_dump.

   The writer returns an error code: 0 means no errors; any other value means
   an error and stops lua_dump from calling the writer again.

     ----------------------------------------------------------------------

  lua_xmove

   [-?, +?, –]

 void lua_xmove (lua_State *from, lua_State *to, int n);

   Exchange values between different threads of the same state.

   This function pops n values from the stack from, and pushes them onto the
   stack to.

     ----------------------------------------------------------------------

  lua_yield

   [-?, +?, v]

 int lua_yield (lua_State *L, int nresults);

   This function is equivalent to lua_yieldk, but it has no continuation (see
   §4.5). Therefore, when the thread resumes, it continues the function that
   called the function calling lua_yield. To avoid surprises, this function
   should be called only in a tail call.

     ----------------------------------------------------------------------

  lua_yieldk

   [-?, +?, v]

 int lua_yieldk (lua_State *L,
                 int nresults,
                 lua_KContext ctx,
                 lua_KFunction k);

   Yields a coroutine (thread).

   When a C function calls lua_yieldk, the running coroutine suspends its
   execution, and the call to lua_resume that started this coroutine returns.
   The parameter nresults is the number of values from the stack that will be
   passed as results to lua_resume.

   When the coroutine is resumed again, Lua calls the given continuation
   function k to continue the execution of the C function that yielded (see
   §4.5). This continuation function receives the same stack from the
   previous function, with the n results removed and replaced by the
   arguments passed to lua_resume. Moreover, the continuation function
   receives the value ctx that was passed to lua_yieldk.

   Usually, this function does not return; when the coroutine eventually
   resumes, it continues executing the continuation function. However, there
   is one special case, which is when this function is called from inside a
   line or a count hook (see §4.7). In that case, lua_yieldk should be called
   with no continuation (probably in the form of lua_yield) and no results,
   and the hook should return immediately after the call. Lua will yield and,
   when the coroutine resumes again, it will continue the normal execution of
   the (Lua) function that triggered the hook.

   This function can raise an error if it is called from a thread with a
   pending C call with no continuation function (what is called a C-call
   boundary), or it is called from a thread that is not running inside a
   resume (typically the main thread).

4.7 – The Debug Interface

   Lua has no built-in debugging facilities. Instead, it offers a special
   interface by means of functions and hooks. This interface allows the
   construction of different kinds of debuggers, profilers, and other tools
   that need "inside information" from the interpreter.

     ----------------------------------------------------------------------

  lua_Debug

 typedef struct lua_Debug {
   int event;
   const char *name;           /* (n) */
   const char *namewhat;       /* (n) */
   const char *what;           /* (S) */
   const char *source;         /* (S) */
   size_t srclen;              /* (S) */
   int currentline;            /* (l) */
   int linedefined;            /* (S) */
   int lastlinedefined;        /* (S) */
   unsigned char nups;         /* (u) number of upvalues */
   unsigned char nparams;      /* (u) number of parameters */
   char isvararg;              /* (u) */
   char istailcall;            /* (t) */
   unsigned short ftransfer;   /* (r) index of first value transferred */
   unsigned short ntransfer;   /* (r) number of transferred values */
   char short_src[LUA_IDSIZE]; /* (S) */
   /* private part */
   other fields
 } lua_Debug;

   A structure used to carry different pieces of information about a function
   or an activation record. lua_getstack fills only the private part of this
   structure, for later use. To fill the other fields of lua_Debug with
   useful information, you must call lua_getinfo with an appropriate
   parameter. (Specifically, to get a field, you must add the letter between
   parentheses in the field's comment to the parameter what of lua_getinfo.)

   The fields of lua_Debug have the following meaning:
     * source: the source of the chunk that created the function. If source
       starts with a '@', it means that the function was defined in a file
       where the file name follows the '@'. If source starts with a '=', the
       remainder of its contents describes the source in a user-dependent
       manner. Otherwise, the function was defined in a string where source
       is that string.
     * srclen: The length of the string source.
     * short_src: a "printable" version of source, to be used in error
       messages.
     * linedefined: the line number where the definition of the function
       starts.
     * lastlinedefined: the line number where the definition of the function
       ends.
     * what: the string "Lua" if the function is a Lua function, "C" if it is
       a C function, "main" if it is the main part of a chunk.
     * currentline: the current line where the given function is executing.
       When no line information is available, currentline is set to -1.
     * name: a reasonable name for the given function. Because functions in
       Lua are first-class values, they do not have a fixed name: some
       functions can be the value of multiple global variables, while others
       can be stored only in a table field. The lua_getinfo function checks
       how the function was called to find a suitable name. If it cannot find
       a name, then name is set to NULL.
     * namewhat: explains the name field. The value of namewhat can be
       "global", "local", "method", "field", "upvalue", or "" (the empty
       string), according to how the function was called. (Lua uses the empty
       string when no other option seems to apply.)
     * istailcall: true if this function invocation was called by a tail
       call. In this case, the caller of this level is not in the stack.
     * nups: the number of upvalues of the function.
     * nparams: the number of parameters of the function (always 0 for
       C functions).
     * isvararg: true if the function is a vararg function (always true for
       C functions).
     * ftransfer: the index in the stack of the first value being
       "transferred", that is, parameters in a call or return values in a
       return. (The other values are in consecutive indices.) Using this
       index, you can access and modify these values through lua_getlocal and
       lua_setlocal. This field is only meaningful during a call hook,
       denoting the first parameter, or a return hook, denoting the first
       value being returned. (For call hooks, this value is always 1.)
     * ntransfer: The number of values being transferred (see previous item).
       (For calls of Lua functions, this value is always equal to nparams.)

     ----------------------------------------------------------------------

  lua_gethook

   [-0, +0, –]

 lua_Hook lua_gethook (lua_State *L);

   Returns the current hook function.

     ----------------------------------------------------------------------

  lua_gethookcount

   [-0, +0, –]

 int lua_gethookcount (lua_State *L);

   Returns the current hook count.

     ----------------------------------------------------------------------

  lua_gethookmask

   [-0, +0, –]

 int lua_gethookmask (lua_State *L);

   Returns the current hook mask.

     ----------------------------------------------------------------------

  lua_getinfo

   [-(0|1), +(0|1|2), m]

 int lua_getinfo (lua_State *L, const char *what, lua_Debug *ar);

   Gets information about a specific function or function invocation.

   To get information about a function invocation, the parameter ar must be a
   valid activation record that was filled by a previous call to lua_getstack
   or given as argument to a hook (see lua_Hook).

   To get information about a function, you push it onto the stack and start
   the what string with the character '>'. (In that case, lua_getinfo pops
   the function from the top of the stack.) For instance, to know in which
   line a function f was defined, you can write the following code:

      lua_Debug ar;
      lua_getglobal(L, "f");  /* get global 'f' */
      lua_getinfo(L, ">S", &ar);
      printf("%d\n", ar.linedefined);

   Each character in the string what selects some fields of the structure ar
   to be filled or a value to be pushed on the stack. (These characters are
   also documented in the declaration of the structure lua_Debug, between
   parentheses in the comments following each field.)
     * 'f': pushes onto the stack the function that is running at the given
       level;
     * 'l': fills in the field currentline;
     * 'n': fills in the fields name and namewhat;
     * 'r': fills in the fields ftransfer and ntransfer;
     * 'S': fills in the fields source, short_src, linedefined,
       lastlinedefined, and what;
     * 't': fills in the field istailcall;
     * 'u': fills in the fields nups, nparams, and isvararg;
     * 'L': pushes onto the stack a table whose indices are the lines on the
       function with some associated code, that is, the lines where you can
       put a break point. (Lines with no code include empty lines and
       comments.) If this option is given together with option 'f', its table
       is pushed after the function. This is the only option that can raise a
       memory error.

   This function returns 0 to signal an invalid option in what; even then the
   valid options are handled correctly.

     ----------------------------------------------------------------------

  lua_getlocal

   [-0, +(0|1), –]

 const char *lua_getlocal (lua_State *L, const lua_Debug *ar, int n);

   Gets information about a local variable or a temporary value of a given
   activation record or a given function.

   In the first case, the parameter ar must be a valid activation record that
   was filled by a previous call to lua_getstack or given as argument to a
   hook (see lua_Hook). The index n selects which local variable to inspect;
   see debug.getlocal for details about variable indices and names.

   lua_getlocal pushes the variable's value onto the stack and returns its
   name.

   In the second case, ar must be NULL and the function to be inspected must
   be on the top of the stack. In this case, only parameters of Lua functions
   are visible (as there is no information about what variables are active)
   and no values are pushed onto the stack.

   Returns NULL (and pushes nothing) when the index is greater than the
   number of active local variables.

     ----------------------------------------------------------------------

  lua_getstack

   [-0, +0, –]

 int lua_getstack (lua_State *L, int level, lua_Debug *ar);

   Gets information about the interpreter runtime stack.

   This function fills parts of a lua_Debug structure with an identification
   of the activation record of the function executing at a given level.
   Level 0 is the current running function, whereas level n+1 is the function
   that has called level n (except for tail calls, which do not count in the
   stack). When called with a level greater than the stack depth,
   lua_getstack returns 0; otherwise it returns 1.

     ----------------------------------------------------------------------

  lua_getupvalue

   [-0, +(0|1), –]

 const char *lua_getupvalue (lua_State *L, int funcindex, int n);

   Gets information about the n-th upvalue of the closure at index funcindex.
   It pushes the upvalue's value onto the stack and returns its name. Returns
   NULL (and pushes nothing) when the index n is greater than the number of
   upvalues.

   See debug.getupvalue for more information about upvalues.

     ----------------------------------------------------------------------

  lua_Hook

 typedef void (*lua_Hook) (lua_State *L, lua_Debug *ar);

   Type for debugging hook functions.

   Whenever a hook is called, its ar argument has its field event set to the
   specific event that triggered the hook. Lua identifies these events with
   the following constants: LUA_HOOKCALL, LUA_HOOKRET, LUA_HOOKTAILCALL,
   LUA_HOOKLINE, and LUA_HOOKCOUNT. Moreover, for line events, the field
   currentline is also set. To get the value of any other field in ar, the
   hook must call lua_getinfo.

   For call events, event can be LUA_HOOKCALL, the normal value, or
   LUA_HOOKTAILCALL, for a tail call; in this case, there will be no
   corresponding return event.

   While Lua is running a hook, it disables other calls to hooks. Therefore,
   if a hook calls back Lua to execute a function or a chunk, this execution
   occurs without any calls to hooks.

   Hook functions cannot have continuations, that is, they cannot call
   lua_yieldk, lua_pcallk, or lua_callk with a non-null k.

   Hook functions can yield under the following conditions: Only count and
   line events can yield; to yield, a hook function must finish its execution
   calling lua_yield with nresults equal to zero (that is, with no values).

     ----------------------------------------------------------------------

  lua_sethook

   [-0, +0, –]

 void lua_sethook (lua_State *L, lua_Hook f, int mask, int count);

   Sets the debugging hook function.

   Argument f is the hook function. mask specifies on which events the hook
   will be called: it is formed by a bitwise OR of the constants
   LUA_MASKCALL, LUA_MASKRET, LUA_MASKLINE, and LUA_MASKCOUNT. The count
   argument is only meaningful when the mask includes LUA_MASKCOUNT. For each
   event, the hook is called as explained below:
     * The call hook: is called when the interpreter calls a function. The
       hook is called just after Lua enters the new function.
     * The return hook: is called when the interpreter returns from a
       function. The hook is called just before Lua leaves the function.
     * The line hook: is called when the interpreter is about to start the
       execution of a new line of code, or when it jumps back in the code
       (even to the same line). This event only happens while Lua is
       executing a Lua function.
     * The count hook: is called after the interpreter executes every count
       instructions. This event only happens while Lua is executing a Lua
       function.

   Hooks are disabled by setting mask to zero.

     ----------------------------------------------------------------------

  lua_setlocal

   [-(0|1), +0, –]

 const char *lua_setlocal (lua_State *L, const lua_Debug *ar, int n);

   Sets the value of a local variable of a given activation record. It
   assigns the value on the top of the stack to the variable and returns its
   name. It also pops the value from the stack.

   Returns NULL (and pops nothing) when the index is greater than the number
   of active local variables.

   Parameters ar and n are as in the function lua_getlocal.

     ----------------------------------------------------------------------

  lua_setupvalue

   [-(0|1), +0, –]

 const char *lua_setupvalue (lua_State *L, int funcindex, int n);

   Sets the value of a closure's upvalue. It assigns the value on the top of
   the stack to the upvalue and returns its name. It also pops the value from
   the stack.

   Returns NULL (and pops nothing) when the index n is greater than the
   number of upvalues.

   Parameters funcindex and n are as in the function lua_getupvalue.

     ----------------------------------------------------------------------

  lua_upvalueid

   [-0, +0, –]

 void *lua_upvalueid (lua_State *L, int funcindex, int n);

   Returns a unique identifier for the upvalue numbered n from the closure at
   index funcindex.

   These unique identifiers allow a program to check whether different
   closures share upvalues. Lua closures that share an upvalue (that is, that
   access a same external local variable) will return identical ids for those
   upvalue indices.

   Parameters funcindex and n are as in the function lua_getupvalue, but n
   cannot be greater than the number of upvalues.

     ----------------------------------------------------------------------

  lua_upvaluejoin

   [-0, +0, –]

 void lua_upvaluejoin (lua_State *L, int funcindex1, int n1,
                                     int funcindex2, int n2);

   Make the n1-th upvalue of the Lua closure at index funcindex1 refer to the
   n2-th upvalue of the Lua closure at index funcindex2.

                           5 – The Auxiliary Library

   The auxiliary library provides several convenient functions to interface C
   with Lua. While the basic API provides the primitive functions for all
   interactions between C and Lua, the auxiliary library provides
   higher-level functions for some common tasks.

   All functions and types from the auxiliary library are defined in header
   file lauxlib.h and have a prefix luaL_.

   All functions in the auxiliary library are built on top of the basic API,
   and so they provide nothing that cannot be done with that API.
   Nevertheless, the use of the auxiliary library ensures more consistency to
   your code.

   Several functions in the auxiliary library use internally some extra stack
   slots. When a function in the auxiliary library uses less than five slots,
   it does not check the stack size; it simply assumes that there are enough
   slots.

   Several functions in the auxiliary library are used to check C function
   arguments. Because the error message is formatted for arguments (e.g.,
   "bad argument #1"), you should not use these functions for other stack
   values.

   Functions called luaL_check* always raise an error if the check is not
   satisfied.

5.1 – Functions and Types

   Here we list all functions and types from the auxiliary library in
   alphabetical order.

     ----------------------------------------------------------------------

  luaL_addchar

   [-?, +?, m]

 void luaL_addchar (luaL_Buffer *B, char c);

   Adds the byte c to the buffer B (see luaL_Buffer).

     ----------------------------------------------------------------------

  luaL_addgsub

   [-?, +?, m]

 const void luaL_addgsub (luaL_Buffer *B, const char *s,
                          const char *p, const char *r);

   Adds a copy of the string s to the buffer B (see luaL_Buffer), replacing
   any occurrence of the string p with the string r.

     ----------------------------------------------------------------------

  luaL_addlstring

   [-?, +?, m]

 void luaL_addlstring (luaL_Buffer *B, const char *s, size_t l);

   Adds the string pointed to by s with length l to the buffer B (see
   luaL_Buffer). The string can contain embedded zeros.

     ----------------------------------------------------------------------

  luaL_addsize

   [-?, +?, –]

 void luaL_addsize (luaL_Buffer *B, size_t n);

   Adds to the buffer B a string of length n previously copied to the buffer
   area (see luaL_prepbuffer).

     ----------------------------------------------------------------------

  luaL_addstring

   [-?, +?, m]

 void luaL_addstring (luaL_Buffer *B, const char *s);

   Adds the zero-terminated string pointed to by s to the buffer B (see
   luaL_Buffer).

     ----------------------------------------------------------------------

  luaL_addvalue

   [-?, +?, m]

 void luaL_addvalue (luaL_Buffer *B);

   Adds the value on the top of the stack to the buffer B (see luaL_Buffer).
   Pops the value.

   This is the only function on string buffers that can (and must) be called
   with an extra element on the stack, which is the value to be added to the
   buffer.

     ----------------------------------------------------------------------

  luaL_argcheck

   [-0, +0, v]

 void luaL_argcheck (lua_State *L,
                     int cond,
                     int arg,
                     const char *extramsg);

   Checks whether cond is true. If it is not, raises an error with a standard
   message (see luaL_argerror).

     ----------------------------------------------------------------------

  luaL_argerror

   [-0, +0, v]

 int luaL_argerror (lua_State *L, int arg, const char *extramsg);

   Raises an error reporting a problem with argument arg of the C function
   that called it, using a standard message that includes extramsg as a
   comment:

      bad argument #arg to 'funcname' (extramsg)

   This function never returns.

     ----------------------------------------------------------------------

  luaL_argexpected

   [-0, +0, v]

 void luaL_argexpected (lua_State *L,
                        int cond,
                        int arg,
                        const char *tname);

   Checks whether cond is true. If it is not, raises an error about the type
   of the argument arg with a standard message (see luaL_typeerror).

     ----------------------------------------------------------------------

  luaL_Buffer

 typedef struct luaL_Buffer luaL_Buffer;

   Type for a string buffer.

   A string buffer allows C code to build Lua strings piecemeal. Its pattern
   of use is as follows:
     * First declare a variable b of type luaL_Buffer.
     * Then initialize it with a call luaL_buffinit(L, &b).
     * Then add string pieces to the buffer calling any of the luaL_add*
       functions.
     * Finish by calling luaL_pushresult(&b). This call leaves the final
       string on the top of the stack.

   If you know beforehand the maximum size of the resulting string, you can
   use the buffer like this:
     * First declare a variable b of type luaL_Buffer.
     * Then initialize it and preallocate a space of size sz with a call
       luaL_buffinitsize(L, &b, sz).
     * Then produce the string into that space.
     * Finish by calling luaL_pushresultsize(&b, sz), where sz is the total
       size of the resulting string copied into that space (which may be less
       than or equal to the preallocated size).

   During its normal operation, a string buffer uses a variable number of
   stack slots. So, while using a buffer, you cannot assume that you know
   where the top of the stack is. You can use the stack between successive
   calls to buffer operations as long as that use is balanced; that is, when
   you call a buffer operation, the stack is at the same level it was
   immediately after the previous buffer operation. (The only exception to
   this rule is luaL_addvalue.) After calling luaL_pushresult, the stack is
   back to its level when the buffer was initialized, plus the final string
   on its top.

     ----------------------------------------------------------------------

  luaL_buffaddr

   [-0, +0, –]

 char *luaL_buffaddr (luaL_Buffer *B);

   Returns the address of the current content of buffer B (see luaL_Buffer).
   Note that any addition to the buffer may invalidate this address.

     ----------------------------------------------------------------------

  luaL_buffinit

   [-0, +?, –]

 void luaL_buffinit (lua_State *L, luaL_Buffer *B);

   Initializes a buffer B (see luaL_Buffer). This function does not allocate
   any space; the buffer must be declared as a variable.

     ----------------------------------------------------------------------

  luaL_bufflen

   [-0, +0, –]

 size_t luaL_bufflen (luaL_Buffer *B);

   Returns the length of the current content of buffer B (see luaL_Buffer).

     ----------------------------------------------------------------------

  luaL_buffinitsize

   [-?, +?, m]

 char *luaL_buffinitsize (lua_State *L, luaL_Buffer *B, size_t sz);

   Equivalent to the sequence luaL_buffinit, luaL_prepbuffsize.

     ----------------------------------------------------------------------

  luaL_buffsub

   [-?, +?, –]

 void luaL_buffsub (luaL_Buffer *B, int n);

   Removes n bytes from the the buffer B (see luaL_Buffer). The buffer must
   have at least that many bytes.

     ----------------------------------------------------------------------

  luaL_callmeta

   [-0, +(0|1), e]

 int luaL_callmeta (lua_State *L, int obj, const char *e);

   Calls a metamethod.

   If the object at index obj has a metatable and this metatable has a field
   e, this function calls this field passing the object as its only argument.
   In this case this function returns true and pushes onto the stack the
   value returned by the call. If there is no metatable or no metamethod,
   this function returns false without pushing any value on the stack.

     ----------------------------------------------------------------------

  luaL_checkany

   [-0, +0, v]

 void luaL_checkany (lua_State *L, int arg);

   Checks whether the function has an argument of any type (including nil) at
   position arg.

     ----------------------------------------------------------------------

  luaL_checkinteger

   [-0, +0, v]

 lua_Integer luaL_checkinteger (lua_State *L, int arg);

   Checks whether the function argument arg is an integer (or can be
   converted to an integer) and returns this integer.

     ----------------------------------------------------------------------

  luaL_checklstring

   [-0, +0, v]

 const char *luaL_checklstring (lua_State *L, int arg, size_t *l);

   Checks whether the function argument arg is a string and returns this
   string; if l is not NULL fills its referent with the string's length.

   This function uses lua_tolstring to get its result, so all conversions and
   caveats of that function apply here.

     ----------------------------------------------------------------------

  luaL_checknumber

   [-0, +0, v]

 lua_Number luaL_checknumber (lua_State *L, int arg);

   Checks whether the function argument arg is a number and returns this
   number converted to a lua_Number.

     ----------------------------------------------------------------------

  luaL_checkoption

   [-0, +0, v]

 int luaL_checkoption (lua_State *L,
                       int arg,
                       const char *def,
                       const char *const lst[]);

   Checks whether the function argument arg is a string and searches for this
   string in the array lst (which must be NULL-terminated). Returns the index
   in the array where the string was found. Raises an error if the argument
   is not a string or if the string cannot be found.

   If def is not NULL, the function uses def as a default value when there is
   no argument arg or when this argument is nil.

   This is a useful function for mapping strings to C enums. (The usual
   convention in Lua libraries is to use strings instead of numbers to select
   options.)

     ----------------------------------------------------------------------

  luaL_checkstack

   [-0, +0, v]

 void luaL_checkstack (lua_State *L, int sz, const char *msg);

   Grows the stack size to top + sz elements, raising an error if the stack
   cannot grow to that size. msg is an additional text to go into the error
   message (or NULL for no additional text).

     ----------------------------------------------------------------------

  luaL_checkstring

   [-0, +0, v]

 const char *luaL_checkstring (lua_State *L, int arg);

   Checks whether the function argument arg is a string and returns this
   string.

   This function uses lua_tolstring to get its result, so all conversions and
   caveats of that function apply here.

     ----------------------------------------------------------------------

  luaL_checktype

   [-0, +0, v]

 void luaL_checktype (lua_State *L, int arg, int t);

   Checks whether the function argument arg has type t. See lua_type for the
   encoding of types for t.

     ----------------------------------------------------------------------

  luaL_checkudata

   [-0, +0, v]

 void *luaL_checkudata (lua_State *L, int arg, const char *tname);

   Checks whether the function argument arg is a userdata of the type tname
   (see luaL_newmetatable) and returns the userdata's memory-block address
   (see lua_touserdata).

     ----------------------------------------------------------------------

  luaL_checkversion

   [-0, +0, v]

 void luaL_checkversion (lua_State *L);

   Checks whether the code making the call and the Lua library being called
   are using the same version of Lua and the same numeric types.

     ----------------------------------------------------------------------

  luaL_dofile

   [-0, +?, m]

 int luaL_dofile (lua_State *L, const char *filename);

   Loads and runs the given file. It is defined as the following macro:

      (luaL_loadfile(L, filename) || lua_pcall(L, 0, LUA_MULTRET, 0))

   It returns LUA_OK if there are no errors, or an error code in case of
   errors (see §4.4.1).

     ----------------------------------------------------------------------

  luaL_dostring

   [-0, +?, –]

 int luaL_dostring (lua_State *L, const char *str);

   Loads and runs the given string. It is defined as the following macro:

      (luaL_loadstring(L, str) || lua_pcall(L, 0, LUA_MULTRET, 0))

   It returns LUA_OK if there are no errors, or an error code in case of
   errors (see §4.4.1).

     ----------------------------------------------------------------------

  luaL_error

   [-0, +0, v]

 int luaL_error (lua_State *L, const char *fmt, ...);

   Raises an error. The error message format is given by fmt plus any extra
   arguments, following the same rules of lua_pushfstring. It also adds at
   the beginning of the message the file name and the line number where the
   error occurred, if this information is available.

   This function never returns, but it is an idiom to use it in C functions
   as return luaL_error(args).

     ----------------------------------------------------------------------

  luaL_execresult

   [-0, +3, m]

 int luaL_execresult (lua_State *L, int stat);

   This function produces the return values for process-related functions in
   the standard library (os.execute and io.close).

     ----------------------------------------------------------------------

  luaL_fileresult

   [-0, +(1|3), m]

 int luaL_fileresult (lua_State *L, int stat, const char *fname);

   This function produces the return values for file-related functions in the
   standard library (io.open, os.rename, file:seek, etc.).

     ----------------------------------------------------------------------

  luaL_getmetafield

   [-0, +(0|1), m]

 int luaL_getmetafield (lua_State *L, int obj, const char *e);

   Pushes onto the stack the field e from the metatable of the object at
   index obj and returns the type of the pushed value. If the object does not
   have a metatable, or if the metatable does not have this field, pushes
   nothing and returns LUA_TNIL.

     ----------------------------------------------------------------------

  luaL_getmetatable

   [-0, +1, m]

 int luaL_getmetatable (lua_State *L, const char *tname);

   Pushes onto the stack the metatable associated with the name tname in the
   registry (see luaL_newmetatable), or nil if there is no metatable
   associated with that name. Returns the type of the pushed value.

     ----------------------------------------------------------------------

  luaL_getsubtable

   [-0, +1, e]

 int luaL_getsubtable (lua_State *L, int idx, const char *fname);

   Ensures that the value t[fname], where t is the value at index idx, is a
   table, and pushes that table onto the stack. Returns true if it finds a
   previous table there and false if it creates a new table.

     ----------------------------------------------------------------------

  luaL_gsub

   [-0, +1, m]

 const char *luaL_gsub (lua_State *L,
                        const char *s,
                        const char *p,
                        const char *r);

   Creates a copy of string s, replacing any occurrence of the string p with
   the string r. Pushes the resulting string on the stack and returns it.

     ----------------------------------------------------------------------

  luaL_len

   [-0, +0, e]

 lua_Integer luaL_len (lua_State *L, int index);

   Returns the "length" of the value at the given index as a number; it is
   equivalent to the '#' operator in Lua (see §3.4.7). Raises an error if the
   result of the operation is not an integer. (This case can only happen
   through metamethods.)

     ----------------------------------------------------------------------

  luaL_loadbuffer

   [-0, +1, –]

 int luaL_loadbuffer (lua_State *L,
                      const char *buff,
                      size_t sz,
                      const char *name);

   Equivalent to luaL_loadbufferx with mode equal to NULL.

     ----------------------------------------------------------------------

  luaL_loadbufferx

   [-0, +1, –]

 int luaL_loadbufferx (lua_State *L,
                       const char *buff,
                       size_t sz,
                       const char *name,
                       const char *mode);

   Loads a buffer as a Lua chunk. This function uses lua_load to load the
   chunk in the buffer pointed to by buff with size sz.

   This function returns the same results as lua_load. name is the chunk
   name, used for debug information and error messages. The string mode works
   as in the function lua_load.

     ----------------------------------------------------------------------

  luaL_loadfile

   [-0, +1, m]

 int luaL_loadfile (lua_State *L, const char *filename);

   Equivalent to luaL_loadfilex with mode equal to NULL.

     ----------------------------------------------------------------------

  luaL_loadfilex

   [-0, +1, m]

 int luaL_loadfilex (lua_State *L, const char *filename,
                                             const char *mode);

   Loads a file as a Lua chunk. This function uses lua_load to load the chunk
   in the file named filename. If filename is NULL, then it loads from the
   standard input. The first line in the file is ignored if it starts with a
   #.

   The string mode works as in the function lua_load.

   This function returns the same results as lua_load or LUA_ERRFILE for
   file-related errors.

   As lua_load, this function only loads the chunk; it does not run it.

     ----------------------------------------------------------------------

  luaL_loadstring

   [-0, +1, –]

 int luaL_loadstring (lua_State *L, const char *s);

   Loads a string as a Lua chunk. This function uses lua_load to load the
   chunk in the zero-terminated string s.

   This function returns the same results as lua_load.

   Also as lua_load, this function only loads the chunk; it does not run it.

     ----------------------------------------------------------------------

  luaL_newlib

   [-0, +1, m]

 void luaL_newlib (lua_State *L, const luaL_Reg l[]);

   Creates a new table and registers there the functions in the list l.

   It is implemented as the following macro:

      (luaL_newlibtable(L,l), luaL_setfuncs(L,l,0))

   The array l must be the actual array, not a pointer to it.

     ----------------------------------------------------------------------

  luaL_newlibtable

   [-0, +1, m]

 void luaL_newlibtable (lua_State *L, const luaL_Reg l[]);

   Creates a new table with a size optimized to store all entries in the
   array l (but does not actually store them). It is intended to be used in
   conjunction with luaL_setfuncs (see luaL_newlib).

   It is implemented as a macro. The array l must be the actual array, not a
   pointer to it.

     ----------------------------------------------------------------------

  luaL_newmetatable

   [-0, +1, m]

 int luaL_newmetatable (lua_State *L, const char *tname);

   If the registry already has the key tname, returns 0. Otherwise, creates a
   new table to be used as a metatable for userdata, adds to this new table
   the pair __name = tname, adds to the registry the pair [tname] = new
   table, and returns 1.

   In both cases, the function pushes onto the stack the final value
   associated with tname in the registry.

     ----------------------------------------------------------------------

  luaL_newstate

   [-0, +0, –]

 lua_State *luaL_newstate (void);

   Creates a new Lua state. It calls lua_newstate with an allocator based on
   the standard C allocation functions and then sets a warning function and a
   panic function (see §4.4) that print messages to the standard error
   output.

   Returns the new state, or NULL if there is a memory allocation error.

     ----------------------------------------------------------------------

  luaL_openlibs

   [-0, +0, e]

 void luaL_openlibs (lua_State *L);

   Opens all standard Lua libraries into the given state.

     ----------------------------------------------------------------------

  luaL_opt

   [-0, +0, –]

 T luaL_opt (L, func, arg, dflt);

   This macro is defined as follows:

      (lua_isnoneornil(L,(arg)) ? (dflt) : func(L,(arg)))

   In words, if the argument arg is nil or absent, the macro results in the
   default dflt. Otherwise, it results in the result of calling func with the
   state L and the argument index arg as arguments. Note that it evaluates
   the expression dflt only if needed.

     ----------------------------------------------------------------------

  luaL_optinteger

   [-0, +0, v]

 lua_Integer luaL_optinteger (lua_State *L,
                              int arg,
                              lua_Integer d);

   If the function argument arg is an integer (or it is convertible to an
   integer), returns this integer. If this argument is absent or is nil,
   returns d. Otherwise, raises an error.

     ----------------------------------------------------------------------

  luaL_optlstring

   [-0, +0, v]

 const char *luaL_optlstring (lua_State *L,
                              int arg,
                              const char *d,
                              size_t *l);

   If the function argument arg is a string, returns this string. If this
   argument is absent or is nil, returns d. Otherwise, raises an error.

   If l is not NULL, fills its referent with the result's length. If the
   result is NULL (only possible when returning d and d == NULL), its length
   is considered zero.

   This function uses lua_tolstring to get its result, so all conversions and
   caveats of that function apply here.

     ----------------------------------------------------------------------

  luaL_optnumber

   [-0, +0, v]

 lua_Number luaL_optnumber (lua_State *L, int arg, lua_Number d);

   If the function argument arg is a number, returns this number as a
   lua_Number. If this argument is absent or is nil, returns d. Otherwise,
   raises an error.

     ----------------------------------------------------------------------

  luaL_optstring

   [-0, +0, v]

 const char *luaL_optstring (lua_State *L,
                             int arg,
                             const char *d);

   If the function argument arg is a string, returns this string. If this
   argument is absent or is nil, returns d. Otherwise, raises an error.

     ----------------------------------------------------------------------

  luaL_prepbuffer

   [-?, +?, m]

 char *luaL_prepbuffer (luaL_Buffer *B);

   Equivalent to luaL_prepbuffsize with the predefined size LUAL_BUFFERSIZE.

     ----------------------------------------------------------------------

  luaL_prepbuffsize

   [-?, +?, m]

 char *luaL_prepbuffsize (luaL_Buffer *B, size_t sz);

   Returns an address to a space of size sz where you can copy a string to be
   added to buffer B (see luaL_Buffer). After copying the string into this
   space you must call luaL_addsize with the size of the string to actually
   add it to the buffer.

     ----------------------------------------------------------------------

  luaL_pushfail

   [-0, +1, –]

 void luaL_pushfail (lua_State *L);

   Pushes the fail value onto the stack (see §6).

     ----------------------------------------------------------------------

  luaL_pushresult

   [-?, +1, m]

 void luaL_pushresult (luaL_Buffer *B);

   Finishes the use of buffer B leaving the final string on the top of the
   stack.

     ----------------------------------------------------------------------

  luaL_pushresultsize

   [-?, +1, m]

 void luaL_pushresultsize (luaL_Buffer *B, size_t sz);

   Equivalent to the sequence luaL_addsize, luaL_pushresult.

     ----------------------------------------------------------------------

  luaL_ref

   [-1, +0, m]

 int luaL_ref (lua_State *L, int t);

   Creates and returns a reference, in the table at index t, for the object
   on the top of the stack (and pops the object).

   A reference is a unique integer key. As long as you do not manually add
   integer keys into the table t, luaL_ref ensures the uniqueness of the key
   it returns. You can retrieve an object referred by the reference r by
   calling lua_rawgeti(L, t, r). The function luaL_unref frees a reference.

   If the object on the top of the stack is nil, luaL_ref returns the
   constant LUA_REFNIL. The constant LUA_NOREF is guaranteed to be different
   from any reference returned by luaL_ref.

     ----------------------------------------------------------------------

  luaL_Reg

 typedef struct luaL_Reg {
   const char *name;
   lua_CFunction func;
 } luaL_Reg;

   Type for arrays of functions to be registered by luaL_setfuncs. name is
   the function name and func is a pointer to the function. Any array of
   luaL_Reg must end with a sentinel entry in which both name and func are
   NULL.

     ----------------------------------------------------------------------

  luaL_requiref

   [-0, +1, e]

 void luaL_requiref (lua_State *L, const char *modname,
                     lua_CFunction openf, int glb);

   If package.loaded[modname] is not true, calls the function openf with the
   string modname as an argument and sets the call result to
   package.loaded[modname], as if that function has been called through
   require.

   If glb is true, also stores the module into the global modname.

   Leaves a copy of the module on the stack.

     ----------------------------------------------------------------------

  luaL_setfuncs

   [-nup, +0, m]

 void luaL_setfuncs (lua_State *L, const luaL_Reg *l, int nup);

   Registers all functions in the array l (see luaL_Reg) into the table on
   the top of the stack (below optional upvalues, see next).

   When nup is not zero, all functions are created with nup upvalues,
   initialized with copies of the nup values previously pushed on the stack
   on top of the library table. These values are popped from the stack after
   the registration.

   A function with a NULL value represents a placeholder, which is filled
   with false.

     ----------------------------------------------------------------------

  luaL_setmetatable

   [-0, +0, –]

 void luaL_setmetatable (lua_State *L, const char *tname);

   Sets the metatable of the object on the top of the stack as the metatable
   associated with name tname in the registry (see luaL_newmetatable).

     ----------------------------------------------------------------------

  luaL_Stream

 typedef struct luaL_Stream {
   FILE *f;
   lua_CFunction closef;
 } luaL_Stream;

   The standard representation for file handles used by the standard I/O
   library.

   A file handle is implemented as a full userdata, with a metatable called
   LUA_FILEHANDLE (where LUA_FILEHANDLE is a macro with the actual
   metatable's name). The metatable is created by the I/O library (see
   luaL_newmetatable).

   This userdata must start with the structure luaL_Stream; it can contain
   other data after this initial structure. The field f points to the
   corresponding C stream (or it can be NULL to indicate an incompletely
   created handle). The field closef points to a Lua function that will be
   called to close the stream when the handle is closed or collected; this
   function receives the file handle as its sole argument and must return
   either a true value, in case of success, or a false value plus an error
   message, in case of error. Once Lua calls this field, it changes the field
   value to NULL to signal that the handle is closed.

     ----------------------------------------------------------------------

  luaL_testudata

   [-0, +0, m]

 void *luaL_testudata (lua_State *L, int arg, const char *tname);

   This function works like luaL_checkudata, except that, when the test
   fails, it returns NULL instead of raising an error.

     ----------------------------------------------------------------------

  luaL_tolstring

   [-0, +1, e]

 const char *luaL_tolstring (lua_State *L, int idx, size_t *len);

   Converts any Lua value at the given index to a C string in a reasonable
   format. The resulting string is pushed onto the stack and also returned by
   the function (see §4.1.3). If len is not NULL, the function also sets *len
   with the string length.

   If the value has a metatable with a __tostring field, then luaL_tolstring
   calls the corresponding metamethod with the value as argument, and uses
   the result of the call as its result.

     ----------------------------------------------------------------------

  luaL_traceback

   [-0, +1, m]

 void luaL_traceback (lua_State *L, lua_State *L1, const char *msg,
                      int level);

   Creates and pushes a traceback of the stack L1. If msg is not NULL, it is
   appended at the beginning of the traceback. The level parameter tells at
   which level to start the traceback.

     ----------------------------------------------------------------------

  luaL_typeerror

   [-0, +0, v]

 const char *luaL_typeerror (lua_State *L,
                                       int arg,
                                       const char *tname);

   Raises a type error for the argument arg of the C function that called it,
   using a standard message; tname is a "name" for the expected type. This
   function never returns.

     ----------------------------------------------------------------------

  luaL_typename

   [-0, +0, –]

 const char *luaL_typename (lua_State *L, int index);

   Returns the name of the type of the value at the given index.

     ----------------------------------------------------------------------

  luaL_unref

   [-0, +0, –]

 void luaL_unref (lua_State *L, int t, int ref);

   Releases the reference ref from the table at index t (see luaL_ref). The
   entry is removed from the table, so that the referred object can be
   collected. The reference ref is also freed to be used again.

   If ref is LUA_NOREF or LUA_REFNIL, luaL_unref does nothing.

     ----------------------------------------------------------------------

  luaL_where

   [-0, +1, m]

 void luaL_where (lua_State *L, int lvl);

   Pushes onto the stack a string identifying the current position of the
   control at level lvl in the call stack. Typically this string has the
   following format:

      chunkname:currentline:

   Level 0 is the running function, level 1 is the function that called the
   running function, etc.

   This function is used to build a prefix for error messages.

                           6 – The Standard Libraries

   The standard Lua libraries provide useful functions that are implemented
   in C through the C API. Some of these functions provide essential services
   to the language (e.g., type and getmetatable); others provide access to
   outside services (e.g., I/O); and others could be implemented in Lua
   itself, but that for different reasons deserve an implementation in C
   (e.g., table.sort).

   All libraries are implemented through the official C API and are provided
   as separate C modules. Unless otherwise noted, these library functions do
   not adjust its number of arguments to its expected parameters. For
   instance, a function documented as foo(arg) should not be called without
   an argument.

   The notation fail means a false value representing some kind of failure.
   (Currently, fail is equal to nil, but that may change in future versions.
   The recommendation is to always test the success of these functions with
   (not status), instead of (status == nil).)

   Currently, Lua has the following standard libraries:
     * basic library (§6.1);
     * coroutine library (§6.2);
     * package library (§6.3);
     * string manipulation (§6.4);
     * basic UTF-8 support (§6.5);
     * table manipulation (§6.6);
     * mathematical functions (§6.7) (sin, log, etc.);
     * input and output (§6.8);
     * operating system facilities (§6.9);
     * debug facilities (§6.10).

   Except for the basic and the package libraries, each library provides all
   its functions as fields of a global table or as methods of its objects.

   To have access to these libraries, the C host program should call the
   luaL_openlibs function, which opens all standard libraries. Alternatively,
   the host program can open them individually by using luaL_requiref to call
   luaopen_base (for the basic library), luaopen_package (for the package
   library), luaopen_coroutine (for the coroutine library), luaopen_string
   (for the string library), luaopen_utf8 (for the UTF-8 library),
   luaopen_table (for the table library), luaopen_math (for the mathematical
   library), luaopen_io (for the I/O library), luaopen_os (for the operating
   system library), and luaopen_debug (for the debug library). These
   functions are declared in lualib.h.

6.1 – Basic Functions

   The basic library provides core functions to Lua. If you do not include
   this library in your application, you should check carefully whether you
   need to provide implementations for some of its facilities.

     ----------------------------------------------------------------------

  assert (v [, message])

   Raises an error if the value of its argument v is false (i.e., nil or
   false); otherwise, returns all its arguments. In case of error, message is
   the error object; when absent, it defaults to "assertion failed!"

     ----------------------------------------------------------------------

  collectgarbage ([opt [, arg]])

   This function is a generic interface to the garbage collector. It performs
   different functions according to its first argument, opt:
     * "collect": Performs a full garbage-collection cycle. This is the
       default option.
     * "stop": Stops automatic execution of the garbage collector. The
       collector will run only when explicitly invoked, until a call to
       restart it.
     * "restart": Restarts automatic execution of the garbage collector.
     * "count": Returns the total memory in use by Lua in Kbytes. The value
       has a fractional part, so that it multiplied by 1024 gives the exact
       number of bytes in use by Lua.
     * "step": Performs a garbage-collection step. The step "size" is
       controlled by arg. With a zero value, the collector will perform one
       basic (indivisible) step. For non-zero values, the collector will
       perform as if that amount of memory (in Kbytes) had been allocated by
       Lua. Returns true if the step finished a collection cycle.
     * "isrunning": Returns a boolean that tells whether the collector is
       running (i.e., not stopped).
     * "incremental": Change the collector mode to incremental. This option
       can be followed by three numbers: the garbage-collector pause, the
       step multiplier, and the step size (see §2.5.1). A zero means to not
       change that value.
     * "generational": Change the collector mode to generational. This option
       can be followed by two numbers: the garbage-collector minor multiplier
       and the major multiplier (see §2.5.2). A zero means to not change that
       value.

   See §2.5 for more details about garbage collection and some of these
   options.

   This function should not be called by a finalizer.

     ----------------------------------------------------------------------

  dofile ([filename])

   Opens the named file and executes its content as a Lua chunk. When called
   without arguments, dofile executes the content of the standard input
   (stdin). Returns all values returned by the chunk. In case of errors,
   dofile propagates the error to its caller. (That is, dofile does not run
   in protected mode.)

     ----------------------------------------------------------------------

  error (message [, level])

   Raises an error (see §2.3) with message as the error object. This function
   never returns.

   Usually, error adds some information about the error position at the
   beginning of the message, if the message is a string. The level argument
   specifies how to get the error position. With level 1 (the default), the
   error position is where the error function was called. Level 2 points the
   error to where the function that called error was called; and so on.
   Passing a level 0 avoids the addition of error position information to the
   message.

     ----------------------------------------------------------------------

  _G

   A global variable (not a function) that holds the global environment (see
   §2.2). Lua itself does not use this variable; changing its value does not
   affect any environment, nor vice versa.

     ----------------------------------------------------------------------

  getmetatable (object)

   If object does not have a metatable, returns nil. Otherwise, if the
   object's metatable has a __metatable field, returns the associated value.
   Otherwise, returns the metatable of the given object.

     ----------------------------------------------------------------------

  ipairs (t)

   Returns three values (an iterator function, the table t, and 0) so that
   the construction

      for i,v in ipairs(t) do body end

   will iterate over the key–value pairs (1,t[1]), (2,t[2]), ..., up to the
   first absent index.

     ----------------------------------------------------------------------

  load (chunk [, chunkname [, mode [, env]]])

   Loads a chunk.

   If chunk is a string, the chunk is this string. If chunk is a function,
   load calls it repeatedly to get the chunk pieces. Each call to chunk must
   return a string that concatenates with previous results. A return of an
   empty string, nil, or no value signals the end of the chunk.

   If there are no syntactic errors, load returns the compiled chunk as a
   function; otherwise, it returns fail plus the error message.

   When you load a main chunk, the resulting function will always have
   exactly one upvalue, the _ENV variable (see §2.2). However, when you load
   a binary chunk created from a function (see string.dump), the resulting
   function can have an arbitrary number of upvalues, and there is no
   guarantee that its first upvalue will be the _ENV variable. (A non-main
   function may not even have an _ENV upvalue.)

   Regardless, if the resulting function has any upvalues, its first upvalue
   is set to the value of env, if that parameter is given, or to the value of
   the global environment. Other upvalues are initialized with nil. All
   upvalues are fresh, that is, they are not shared with any other function.

   chunkname is used as the name of the chunk for error messages and debug
   information (see §4.7). When absent, it defaults to chunk, if chunk is a
   string, or to "=(load)" otherwise.

   The string mode controls whether the chunk can be text or binary (that is,
   a precompiled chunk). It may be the string "b" (only binary chunks), "t"
   (only text chunks), or "bt" (both binary and text). The default is "bt".

   It is safe to load malformed binary chunks; load signals an appropriate
   error. However, Lua does not check the consistency of the code inside
   binary chunks; running maliciously crafted bytecode can crash the
   interpreter.

     ----------------------------------------------------------------------

  loadfile ([filename [, mode [, env]]])

   Similar to load, but gets the chunk from file filename or from the
   standard input, if no file name is given.

     ----------------------------------------------------------------------

  next (table [, index])

   Allows a program to traverse all fields of a table. Its first argument is
   a table and its second argument is an index in this table. A call to next
   returns the next index of the table and its associated value. When called
   with nil as its second argument, next returns an initial index and its
   associated value. When called with the last index, or with nil in an empty
   table, next returns nil. If the second argument is absent, then it is
   interpreted as nil. In particular, you can use next(t) to check whether a
   table is empty.

   The order in which the indices are enumerated is not specified, even for
   numeric indices. (To traverse a table in numerical order, use a numerical
   for.)

   You should not assign any value to a non-existent field in a table during
   its traversal. You may however modify existing fields. In particular, you
   may set existing fields to nil.

     ----------------------------------------------------------------------

  pairs (t)

   If t has a metamethod __pairs, calls it with t as argument and returns the
   first three results from the call.

   Otherwise, returns three values: the next function, the table t, and nil,
   so that the construction

      for k,v in pairs(t) do body end

   will iterate over all key–value pairs of table t.

   See function next for the caveats of modifying the table during its
   traversal.

     ----------------------------------------------------------------------

  pcall (f [, arg1, ···])

   Calls the function f with the given arguments in protected mode. This
   means that any error inside f is not propagated; instead, pcall catches
   the error and returns a status code. Its first result is the status code
   (a boolean), which is true if the call succeeds without errors. In such
   case, pcall also returns all results from the call, after this first
   result. In case of any error, pcall returns false plus the error object.
   Note that errors caught by pcall do not call a message handler.

     ----------------------------------------------------------------------

  print (···)

   Receives any number of arguments and prints their values to stdout,
   converting each argument to a string following the same rules of tostring.

   The function print is not intended for formatted output, but only as a
   quick way to show a value, for instance for debugging. For complete
   control over the output, use string.format and io.write.

     ----------------------------------------------------------------------

  rawequal (v1, v2)

   Checks whether v1 is equal to v2, without invoking the __eq metamethod.
   Returns a boolean.

     ----------------------------------------------------------------------

  rawget (table, index)

   Gets the real value of table[index], without using the __index metavalue.
   table must be a table; index may be any value.

     ----------------------------------------------------------------------

  rawlen (v)

   Returns the length of the object v, which must be a table or a string,
   without invoking the __len metamethod. Returns an integer.

     ----------------------------------------------------------------------

  rawset (table, index, value)

   Sets the real value of table[index] to value, without using the __newindex
   metavalue. table must be a table, index any value different from nil and
   NaN, and value any Lua value.

   This function returns table.

     ----------------------------------------------------------------------

  select (index, ···)

   If index is a number, returns all arguments after argument number index; a
   negative number indexes from the end (-1 is the last argument). Otherwise,
   index must be the string "#", and select returns the total number of extra
   arguments it received.

     ----------------------------------------------------------------------

  setmetatable (table, metatable)

   Sets the metatable for the given table. If metatable is nil, removes the
   metatable of the given table. If the original metatable has a __metatable
   field, raises an error.

   This function returns table.

   To change the metatable of other types from Lua code, you must use the
   debug library (§6.10).

     ----------------------------------------------------------------------

  tonumber (e [, base])

   When called with no base, tonumber tries to convert its argument to a
   number. If the argument is already a number or a string convertible to a
   number, then tonumber returns this number; otherwise, it returns fail.

   The conversion of strings can result in integers or floats, according to
   the lexical conventions of Lua (see §3.1). The string may have leading and
   trailing spaces and a sign.

   When called with base, then e must be a string to be interpreted as an
   integer numeral in that base. The base may be any integer between 2 and
   36, inclusive. In bases above 10, the letter 'A' (in either upper or lower
   case) represents 10, 'B' represents 11, and so forth, with 'Z'
   representing 35. If the string e is not a valid numeral in the given base,
   the function returns fail.

     ----------------------------------------------------------------------

  tostring (v)

   Receives a value of any type and converts it to a string in a
   human-readable format.

   If the metatable of v has a __tostring field, then tostring calls the
   corresponding value with v as argument, and uses the result of the call as
   its result. Otherwise, if the metatable of v has a __name field with a
   string value, tostring may use that string in its final result.

   For complete control of how numbers are converted, use string.format.

     ----------------------------------------------------------------------

  type (v)

   Returns the type of its only argument, coded as a string. The possible
   results of this function are "nil" (a string, not the value nil),
   "number", "string", "boolean", "table", "function", "thread", and
   "userdata".

     ----------------------------------------------------------------------

  _VERSION

   A global variable (not a function) that holds a string containing the
   running Lua version. The current value of this variable is "Lua 5.4".

     ----------------------------------------------------------------------

  warn (msg1, ···)

   Emits a warning with a message composed by the concatenation of all its
   arguments (which should be strings).

   By convention, a one-piece message starting with '@' is intended to be a
   control message, which is a message to the warning system itself. In
   particular, the standard warning function in Lua recognizes the control
   messages "@off", to stop the emission of warnings, and "@on", to (re)start
   the emission; it ignores unknown control messages.

     ----------------------------------------------------------------------

  xpcall (f, msgh [, arg1, ···])

   This function is similar to pcall, except that it sets a new message
   handler msgh.

6.2 – Coroutine Manipulation

   This library comprises the operations to manipulate coroutines, which come
   inside the table coroutine. See §2.6 for a general description of
   coroutines.

     ----------------------------------------------------------------------

  coroutine.close (co)

   Closes coroutine co, that is, closes all its pending to-be-closed
   variables and puts the coroutine in a dead state. The given coroutine must
   be dead or suspended. In case of error (either the original error that
   stopped the coroutine or errors in closing methods), returns false plus
   the error object; otherwise returns true.

     ----------------------------------------------------------------------

  coroutine.create (f)

   Creates a new coroutine, with body f. f must be a function. Returns this
   new coroutine, an object with type "thread".

     ----------------------------------------------------------------------

  coroutine.isyieldable ([co])

   Returns true when the coroutine co can yield. The default for co is the
   running coroutine.

   A coroutine is yieldable if it is not the main thread and it is not inside
   a non-yieldable C function.

     ----------------------------------------------------------------------

  coroutine.resume (co [, val1, ···])

   Starts or continues the execution of coroutine co. The first time you
   resume a coroutine, it starts running its body. The values val1, ... are
   passed as the arguments to the body function. If the coroutine has
   yielded, resume restarts it; the values val1, ... are passed as the
   results from the yield.

   If the coroutine runs without any errors, resume returns true plus any
   values passed to yield (when the coroutine yields) or any values returned
   by the body function (when the coroutine terminates). If there is any
   error, resume returns false plus the error message.

     ----------------------------------------------------------------------

  coroutine.running ()

   Returns the running coroutine plus a boolean, true when the running
   coroutine is the main one.

     ----------------------------------------------------------------------

  coroutine.status (co)

   Returns the status of the coroutine co, as a string: "running", if the
   coroutine is running (that is, it is the one that called status);
   "suspended", if the coroutine is suspended in a call to yield, or if it
   has not started running yet; "normal" if the coroutine is active but not
   running (that is, it has resumed another coroutine); and "dead" if the
   coroutine has finished its body function, or if it has stopped with an
   error.

     ----------------------------------------------------------------------

  coroutine.wrap (f)

   Creates a new coroutine, with body f; f must be a function. Returns a
   function that resumes the coroutine each time it is called. Any arguments
   passed to this function behave as the extra arguments to resume. The
   function returns the same values returned by resume, except the first
   boolean. In case of error, the function closes the coroutine and
   propagates the error.

     ----------------------------------------------------------------------

  coroutine.yield (···)

   Suspends the execution of the calling coroutine. Any arguments to yield
   are passed as extra results to resume.

6.3 – Modules

   The package library provides basic facilities for loading modules in Lua.
   It exports one function directly in the global environment: require.
   Everything else is exported in the table package.

     ----------------------------------------------------------------------

  require (modname)

   Loads the given module. The function starts by looking into the
   package.loaded table to determine whether modname is already loaded. If it
   is, then require returns the value stored at package.loaded[modname]. (The
   absence of a second result in this case signals that this call did not
   have to load the module.) Otherwise, it tries to find a loader for the
   module.

   To find a loader, require is guided by the table package.searchers. Each
   item in this table is a search function, that searches for the module in a
   particular way. By changing this table, we can change how require looks
   for a module. The following explanation is based on the default
   configuration for package.searchers.

   First require queries package.preload[modname]. If it has a value, this
   value (which must be a function) is the loader. Otherwise require searches
   for a Lua loader using the path stored in package.path. If that also
   fails, it searches for a C loader using the path stored in package.cpath.
   If that also fails, it tries an all-in-one loader (see package.searchers).

   Once a loader is found, require calls the loader with two arguments:
   modname and an extra value, a loader data, also returned by the searcher.
   The loader data can be any value useful to the module; for the default
   searchers, it indicates where the loader was found. (For instance, if the
   loader came from a file, this extra value is the file path.) If the loader
   returns any non-nil value, require assigns the returned value to
   package.loaded[modname]. If the loader does not return a non-nil value and
   has not assigned any value to package.loaded[modname], then require
   assigns true to this entry. In any case, require returns the final value
   of package.loaded[modname]. Besides that value, require also returns as a
   second result the loader data returned by the searcher, which indicates
   how require found the module.

   If there is any error loading or running the module, or if it cannot find
   any loader for the module, then require raises an error.

     ----------------------------------------------------------------------

  package.config

   A string describing some compile-time configurations for packages. This
   string is a sequence of lines:
     * The first line is the directory separator string. Default is '\' for
       Windows and '/' for all other systems.
     * The second line is the character that separates templates in a path.
       Default is ';'.
     * The third line is the string that marks the substitution points in a
       template. Default is '?'.
     * The fourth line is a string that, in a path in Windows, is replaced by
       the executable's directory. Default is '!'.
     * The fifth line is a mark to ignore all text after it when building the
       luaopen_ function name. Default is '-'.

     ----------------------------------------------------------------------

  package.cpath

   A string with the path used by require to search for a C loader.

   Lua initializes the C path package.cpath in the same way it initializes
   the Lua path package.path, using the environment variable LUA_CPATH_5_4,
   or the environment variable LUA_CPATH, or a default path defined in
   luaconf.h.

     ----------------------------------------------------------------------

  package.loaded

   A table used by require to control which modules are already loaded. When
   you require a module modname and package.loaded[modname] is not false,
   require simply returns the value stored there.

   This variable is only a reference to the real table; assignments to this
   variable do not change the table used by require.

     ----------------------------------------------------------------------

  package.loadlib (libname, funcname)

   Dynamically links the host program with the C library libname.

   If funcname is "*", then it only links with the library, making the
   symbols exported by the library available to other dynamically linked
   libraries. Otherwise, it looks for a function funcname inside the library
   and returns this function as a C function. So, funcname must follow the
   lua_CFunction prototype (see lua_CFunction).

   This is a low-level function. It completely bypasses the package and
   module system. Unlike require, it does not perform any path searching and
   does not automatically adds extensions. libname must be the complete file
   name of the C library, including if necessary a path and an extension.
   funcname must be the exact name exported by the C library (which may
   depend on the C compiler and linker used).

   This function is not supported by Standard C. As such, it is only
   available on some platforms (Windows, Linux, Mac OS X, Solaris, BSD, plus
   other Unix systems that support the dlfcn standard).

   This function is inherently insecure, as it allows Lua to call any
   function in any readable dynamic library in the system. (Lua calls any
   function assuming the function has a proper prototype and respects a
   proper protocol (see lua_CFunction). Therefore, calling an arbitrary
   function in an arbitrary dynamic library more often than not results in an
   access violation.)

     ----------------------------------------------------------------------

  package.path

   A string with the path used by require to search for a Lua loader.

   At start-up, Lua initializes this variable with the value of the
   environment variable LUA_PATH_5_4 or the environment variable LUA_PATH or
   with a default path defined in luaconf.h, if those environment variables
   are not defined. A ";;" in the value of the environment variable is
   replaced by the default path.

     ----------------------------------------------------------------------

  package.preload

   A table to store loaders for specific modules (see require).

   This variable is only a reference to the real table; assignments to this
   variable do not change the table used by require.

     ----------------------------------------------------------------------

  package.searchers

   A table used by require to control how to find modules.

   Each entry in this table is a searcher function. When looking for a
   module, require calls each of these searchers in ascending order, with the
   module name (the argument given to require) as its sole argument. If the
   searcher finds the module, it returns another function, the module loader,
   plus an extra value, a loader data, that will be passed to that loader and
   returned as a second result by require. If it cannot find the module, it
   returns a string explaining why (or nil if it has nothing to say).

   Lua initializes this table with four searcher functions.

   The first searcher simply looks for a loader in the package.preload table.

   The second searcher looks for a loader as a Lua library, using the path
   stored at package.path. The search is done as described in function
   package.searchpath.

   The third searcher looks for a loader as a C library, using the path given
   by the variable package.cpath. Again, the search is done as described in
   function package.searchpath. For instance, if the C path is the string

      "./?.so;./?.dll;/usr/local/?/init.so"

   the searcher for module foo will try to open the files ./foo.so,
   ./foo.dll, and /usr/local/foo/init.so, in that order. Once it finds a
   C library, this searcher first uses a dynamic link facility to link the
   application with the library. Then it tries to find a C function inside
   the library to be used as the loader. The name of this C function is the
   string "luaopen_" concatenated with a copy of the module name where each
   dot is replaced by an underscore. Moreover, if the module name has a
   hyphen, its suffix after (and including) the first hyphen is removed. For
   instance, if the module name is a.b.c-v2.1, the function name will be
   luaopen_a_b_c.

   The fourth searcher tries an all-in-one loader. It searches the C path for
   a library for the root name of the given module. For instance, when
   requiring a.b.c, it will search for a C library for a. If found, it looks
   into it for an open function for the submodule; in our example, that would
   be luaopen_a_b_c. With this facility, a package can pack several
   C submodules into one single library, with each submodule keeping its
   original open function.

   All searchers except the first one (preload) return as the extra value the
   file path where the module was found, as returned by package.searchpath.
   The first searcher always returns the string ":preload:".

   Searchers should raise no errors and have no side effects in Lua. (They
   may have side effects in C, for instance by linking the application with a
   library.)

     ----------------------------------------------------------------------

  package.searchpath (name, path [, sep [, rep]])

   Searches for the given name in the given path.

   A path is a string containing a sequence of templates separated by
   semicolons. For each template, the function replaces each interrogation
   mark (if any) in the template with a copy of name wherein all occurrences
   of sep (a dot, by default) were replaced by rep (the system's directory
   separator, by default), and then tries to open the resulting file name.

   For instance, if the path is the string

      "./?.lua;./?.lc;/usr/local/?/init.lua"

   the search for the name foo.a will try to open the files ./foo/a.lua,
   ./foo/a.lc, and /usr/local/foo/a/init.lua, in that order.

   Returns the resulting name of the first file that it can open in read mode
   (after closing the file), or fail plus an error message if none succeeds.
   (This error message lists all file names it tried to open.)

6.4 – String Manipulation

   This library provides generic functions for string manipulation, such as
   finding and extracting substrings, and pattern matching. When indexing a
   string in Lua, the first character is at position 1 (not at 0, as in C).
   Indices are allowed to be negative and are interpreted as indexing
   backwards, from the end of the string. Thus, the last character is at
   position -1, and so on.

   The string library provides all its functions inside the table string. It
   also sets a metatable for strings where the __index field points to the
   string table. Therefore, you can use the string functions in
   object-oriented style. For instance, string.byte(s,i) can be written as
   s:byte(i).

   The string library assumes one-byte character encodings.

     ----------------------------------------------------------------------

  string.byte (s [, i [, j]])

   Returns the internal numeric codes of the characters s[i], s[i+1], ...,
   s[j]. The default value for i is 1; the default value for j is i. These
   indices are corrected following the same rules of function string.sub.

   Numeric codes are not necessarily portable across platforms.

     ----------------------------------------------------------------------

  string.char (···)

   Receives zero or more integers. Returns a string with length equal to the
   number of arguments, in which each character has the internal numeric code
   equal to its corresponding argument.

   Numeric codes are not necessarily portable across platforms.

     ----------------------------------------------------------------------

  string.dump (function [, strip])

   Returns a string containing a binary representation (a binary chunk) of
   the given function, so that a later load on this string returns a copy of
   the function (but with new upvalues). If strip is a true value, the binary
   representation may not include all debug information about the function,
   to save space.

   Functions with upvalues have only their number of upvalues saved. When
   (re)loaded, those upvalues receive fresh instances. (See the load function
   for details about how these upvalues are initialized. You can use the
   debug library to serialize and reload the upvalues of a function in a way
   adequate to your needs.)

     ----------------------------------------------------------------------

  string.find (s, pattern [, init [, plain]])

   Looks for the first match of pattern (see §6.4.1) in the string s. If it
   finds a match, then find returns the indices of s where this occurrence
   starts and ends; otherwise, it returns fail. A third, optional numeric
   argument init specifies where to start the search; its default value is 1
   and can be negative. A true as a fourth, optional argument plain turns off
   the pattern matching facilities, so the function does a plain "find
   substring" operation, with no characters in pattern being considered
   magic.

   If the pattern has captures, then in a successful match the captured
   values are also returned, after the two indices.

     ----------------------------------------------------------------------

  string.format (formatstring, ···)

   Returns a formatted version of its variable number of arguments following
   the description given in its first argument, which must be a string. The
   format string follows the same rules as the ISO C function sprintf. The
   only differences are that the conversion specifiers and modifiers F, n, *,
   h, L, and l are not supported and that there is an extra specifier, q.
   Both width and precision, when present, are limited to two digits.

   The specifier q formats booleans, nil, numbers, and strings in a way that
   the result is a valid constant in Lua source code. Booleans and nil are
   written in the obvious way (true, false, nil). Floats are written in
   hexadecimal, to preserve full precision. A string is written between
   double quotes, using escape sequences when necessary to ensure that it can
   safely be read back by the Lua interpreter. For instance, the call

      string.format('%q', 'a string with "quotes" and \n new line')

   may produce the string:

      "a string with \"quotes\" and \
       new line"

   This specifier does not support modifiers (flags, width, precision).

   The conversion specifiers A, a, E, e, f, G, and g all expect a number as
   argument. The specifiers c, d, i, o, u, X, and x expect an integer. When
   Lua is compiled with a C89 compiler, the specifiers A and a (hexadecimal
   floats) do not support modifiers.

   The specifier s expects a string; if its argument is not a string, it is
   converted to one following the same rules of tostring. If the specifier
   has any modifier, the corresponding string argument should not contain
   embedded zeros.

   The specifier p formats the pointer returned by lua_topointer. That gives
   a unique string identifier for tables, userdata, threads, strings, and
   functions. For other values (numbers, nil, booleans), this specifier
   results in a string representing the pointer NULL.

     ----------------------------------------------------------------------

  string.gmatch (s, pattern [, init])

   Returns an iterator function that, each time it is called, returns the
   next captures from pattern (see §6.4.1) over the string s. If pattern
   specifies no captures, then the whole match is produced in each call. A
   third, optional numeric argument init specifies where to start the search;
   its default value is 1 and can be negative.

   As an example, the following loop will iterate over all the words from
   string s, printing one per line:

      s = "hello world from Lua"
      for w in string.gmatch(s, "%a+") do
        print(w)
      end

   The next example collects all pairs key=value from the given string into a
   table:

      t = {}
      s = "from=world, to=Lua"
      for k, v in string.gmatch(s, "(%w+)=(%w+)") do
        t[k] = v
      end

   For this function, a caret '^' at the start of a pattern does not work as
   an anchor, as this would prevent the iteration.

     ----------------------------------------------------------------------

  string.gsub (s, pattern, repl [, n])

   Returns a copy of s in which all (or the first n, if given) occurrences of
   the pattern (see §6.4.1) have been replaced by a replacement string
   specified by repl, which can be a string, a table, or a function. gsub
   also returns, as its second value, the total number of matches that
   occurred. The name gsub comes from Global SUBstitution.

   If repl is a string, then its value is used for replacement. The
   character % works as an escape character: any sequence in repl of the form
   %d, with d between 1 and 9, stands for the value of the d-th captured
   substring; the sequence %0 stands for the whole match; the sequence %%
   stands for a single %.

   If repl is a table, then the table is queried for every match, using the
   first capture as the key.

   If repl is a function, then this function is called every time a match
   occurs, with all captured substrings passed as arguments, in order.

   In any case, if the pattern specifies no captures, then it behaves as if
   the whole pattern was inside a capture.

   If the value returned by the table query or by the function call is a
   string or a number, then it is used as the replacement string; otherwise,
   if it is false or nil, then there is no replacement (that is, the original
   match is kept in the string).

   Here are some examples:

      x = string.gsub("hello world", "(%w+)", "%1 %1")
      --> x="hello hello world world"
     
      x = string.gsub("hello world", "%w+", "%0 %0", 1)
      --> x="hello hello world"
     
      x = string.gsub("hello world from Lua", "(%w+)%s*(%w+)", "%2 %1")
      --> x="world hello Lua from"
     
      x = string.gsub("home = $HOME, user = $USER", "%$(%w+)", os.getenv)
      --> x="home = /home/roberto, user = roberto"
     
      x = string.gsub("4+5 = $return 4+5$", "%$(.-)%$", function (s)
            return load(s)()
          end)
      --> x="4+5 = 9"
     
      local t = {name="lua", version="5.4"}
      x = string.gsub("$name-$version.tar.gz", "%$(%w+)", t)
      --> x="lua-5.4.tar.gz"

     ----------------------------------------------------------------------

  string.len (s)

   Receives a string and returns its length. The empty string "" has length
   0. Embedded zeros are counted, so "a\000bc\000" has length 5.

     ----------------------------------------------------------------------

  string.lower (s)

   Receives a string and returns a copy of this string with all uppercase
   letters changed to lowercase. All other characters are left unchanged. The
   definition of what an uppercase letter is depends on the current locale.

     ----------------------------------------------------------------------

  string.match (s, pattern [, init])

   Looks for the first match of the pattern (see §6.4.1) in the string s. If
   it finds one, then match returns the captures from the pattern; otherwise
   it returns fail. If pattern specifies no captures, then the whole match is
   returned. A third, optional numeric argument init specifies where to start
   the search; its default value is 1 and can be negative.

     ----------------------------------------------------------------------

  string.pack (fmt, v1, v2, ···)

   Returns a binary string containing the values v1, v2, etc. serialized in
   binary form (packed) according to the format string fmt (see §6.4.2).

     ----------------------------------------------------------------------

  string.packsize (fmt)

   Returns the size of a string resulting from string.pack with the given
   format. The format string cannot have the variable-length options 's' or
   'z' (see §6.4.2).

     ----------------------------------------------------------------------

  string.rep (s, n [, sep])

   Returns a string that is the concatenation of n copies of the string s
   separated by the string sep. The default value for sep is the empty string
   (that is, no separator). Returns the empty string if n is not positive.

   (Note that it is very easy to exhaust the memory of your machine with a
   single call to this function.)

     ----------------------------------------------------------------------

  string.reverse (s)

   Returns a string that is the string s reversed.

     ----------------------------------------------------------------------

  string.sub (s, i [, j])

   Returns the substring of s that starts at i and continues until j; i and j
   can be negative. If j is absent, then it is assumed to be equal to -1
   (which is the same as the string length). In particular, the call
   string.sub(s,1,j) returns a prefix of s with length j, and string.sub(s,
   -i) (for a positive i) returns a suffix of s with length i.

   If, after the translation of negative indices, i is less than 1, it is
   corrected to 1. If j is greater than the string length, it is corrected to
   that length. If, after these corrections, i is greater than j, the
   function returns the empty string.

     ----------------------------------------------------------------------

  string.unpack (fmt, s [, pos])

   Returns the values packed in string s (see string.pack) according to the
   format string fmt (see §6.4.2). An optional pos marks where to start
   reading in s (default is 1). After the read values, this function also
   returns the index of the first unread byte in s.

     ----------------------------------------------------------------------

  string.upper (s)

   Receives a string and returns a copy of this string with all lowercase
   letters changed to uppercase. All other characters are left unchanged. The
   definition of what a lowercase letter is depends on the current locale.

  6.4.1 – Patterns

   Patterns in Lua are described by regular strings, which are interpreted as
   patterns by the pattern-matching functions string.find, string.gmatch,
   string.gsub, and string.match. This section describes the syntax and the
   meaning (that is, what they match) of these strings.

    Character Class:

   A character class is used to represent a set of characters. The following
   combinations are allowed in describing a character class:
     * x: (where x is not one of the magic characters ^$()%.[]*+-?)
       represents the character x itself.
     * .: (a dot) represents all characters.
     * %a: represents all letters.
     * %c: represents all control characters.
     * %d: represents all digits.
     * %g: represents all printable characters except space.
     * %l: represents all lowercase letters.
     * %p: represents all punctuation characters.
     * %s: represents all space characters.
     * %u: represents all uppercase letters.
     * %w: represents all alphanumeric characters.
     * %x: represents all hexadecimal digits.
     * %x: (where x is any non-alphanumeric character) represents the
       character x. This is the standard way to escape the magic characters.
       Any non-alphanumeric character (including all punctuation characters,
       even the non-magical) can be preceded by a '%' to represent itself in
       a pattern.
     * [set]: represents the class which is the union of all characters in
       set. A range of characters can be specified by separating the end
       characters of the range, in ascending order, with a '-'. All classes
       %x described above can also be used as components in set. All other
       characters in set represent themselves. For example, [%w_] (or [_%w])
       represents all alphanumeric characters plus the underscore, [0-7]
       represents the octal digits, and [0-7%l%-] represents the octal digits
       plus the lowercase letters plus the '-' character.

       You can put a closing square bracket in a set by positioning it as the
       first character in the set. You can put a hyphen in a set by
       positioning it as the first or the last character in the set. (You can
       also use an escape for both cases.)

       The interaction between ranges and classes is not defined. Therefore,
       patterns like [%a-z] or [a-%%] have no meaning.
     * [^set]: represents the complement of set, where set is interpreted as
       above.

   For all classes represented by single letters (%a, %c, etc.), the
   corresponding uppercase letter represents the complement of the class. For
   instance, %S represents all non-space characters.

   The definitions of letter, space, and other character groups depend on the
   current locale. In particular, the class [a-z] may not be equivalent to
   %l.

    Pattern Item:

   A pattern item can be
     * a single character class, which matches any single character in the
       class;
     * a single character class followed by '*', which matches sequences of
       zero or more characters in the class. These repetition items will
       always match the longest possible sequence;
     * a single character class followed by '+', which matches sequences of
       one or more characters in the class. These repetition items will
       always match the longest possible sequence;
     * a single character class followed by '-', which also matches sequences
       of zero or more characters in the class. Unlike '*', these repetition
       items will always match the shortest possible sequence;
     * a single character class followed by '?', which matches zero or one
       occurrence of a character in the class. It always matches one
       occurrence if possible;
     * %n, for n between 1 and 9; such item matches a substring equal to the
       n-th captured string (see below);
     * %bxy, where x and y are two distinct characters; such item matches
       strings that start with x, end with y, and where the x and y are
       balanced. This means that, if one reads the string from left to right,
       counting +1 for an x and -1 for a y, the ending y is the first y where
       the count reaches 0. For instance, the item %b() matches expressions
       with balanced parentheses.
     * %f[set], a frontier pattern; such item matches an empty string at any
       position such that the next character belongs to set and the previous
       character does not belong to set. The set set is interpreted as
       previously described. The beginning and the end of the subject are
       handled as if they were the character '\0'.

    Pattern:

   A pattern is a sequence of pattern items. A caret '^' at the beginning of
   a pattern anchors the match at the beginning of the subject string. A '$'
   at the end of a pattern anchors the match at the end of the subject
   string. At other positions, '^' and '$' have no special meaning and
   represent themselves.

    Captures:

   A pattern can contain sub-patterns enclosed in parentheses; they describe
   captures. When a match succeeds, the substrings of the subject string that
   match captures are stored (captured) for future use. Captures are numbered
   according to their left parentheses. For instance, in the pattern
   "(a*(.)%w(%s*))", the part of the string matching "a*(.)%w(%s*)" is stored
   as the first capture, and therefore has number 1; the character matching
   "." is captured with number 2, and the part matching "%s*" has number 3.

   As a special case, the capture () captures the current string position (a
   number). For instance, if we apply the pattern "()aa()" on the string
   "flaaap", there will be two captures: 3 and 5.

    Multiple matches:

   The function string.gsub and the iterator string.gmatch match multiple
   occurrences of the given pattern in the subject. For these functions, a
   new match is considered valid only if it ends at least one byte after the
   end of the previous match. In other words, the pattern machine never
   accepts the empty string as a match immediately after another match. As an
   example, consider the results of the following code:

      > string.gsub("abc", "()a*()", print);
      --> 1   2
      --> 3   3
      --> 4   4

   The second and third results come from Lua matching an empty string after
   'b' and another one after 'c'. Lua does not match an empty string after
   'a', because it would end at the same position of the previous match.

  6.4.2 – Format Strings for Pack and Unpack

   The first argument to string.pack, string.packsize, and string.unpack is a
   format string, which describes the layout of the structure being created
   or read.

   A format string is a sequence of conversion options. The conversion
   options are as follows:
     * <: sets little endian
     * >: sets big endian
     * =: sets native endian
     * ![n]: sets maximum alignment to n (default is native alignment)
     * b: a signed byte (char)
     * B: an unsigned byte (char)
     * h: a signed short (native size)
     * H: an unsigned short (native size)
     * l: a signed long (native size)
     * L: an unsigned long (native size)
     * j: a lua_Integer
     * J: a lua_Unsigned
     * T: a size_t (native size)
     * i[n]: a signed int with n bytes (default is native size)
     * I[n]: an unsigned int with n bytes (default is native size)
     * f: a float (native size)
     * d: a double (native size)
     * n: a lua_Number
     * cn: a fixed-sized string with n bytes
     * z: a zero-terminated string
     * s[n]: a string preceded by its length coded as an unsigned integer
       with n bytes (default is a size_t)
     * x: one byte of padding
     * Xop: an empty item that aligns according to option op (which is
       otherwise ignored)
     * '  ': (space) ignored

   (A "[n]" means an optional integral numeral.) Except for padding, spaces,
   and configurations (options "xX <=>!"), each option corresponds to an
   argument in string.pack or a result in string.unpack.

   For options "!n", "sn", "in", and "In", n can be any integer between 1 and
   16. All integral options check overflows; string.pack checks whether the
   given value fits in the given size; string.unpack checks whether the read
   value fits in a Lua integer. For the unsigned options, Lua integers are
   treated as unsigned values too.

   Any format string starts as if prefixed by "!1=", that is, with maximum
   alignment of 1 (no alignment) and native endianness.

   Native endianness assumes that the whole system is either big or little
   endian. The packing functions will not emulate correctly the behavior of
   mixed-endian formats.

   Alignment works as follows: For each option, the format gets extra padding
   until the data starts at an offset that is a multiple of the minimum
   between the option size and the maximum alignment; this minimum must be a
   power of 2. Options "c" and "z" are not aligned; option "s" follows the
   alignment of its starting integer.

   All padding is filled with zeros by string.pack and ignored by
   string.unpack.

6.5 – UTF-8 Support

   This library provides basic support for UTF-8 encoding. It provides all
   its functions inside the table utf8. This library does not provide any
   support for Unicode other than the handling of the encoding. Any operation
   that needs the meaning of a character, such as character classification,
   is outside its scope.

   Unless stated otherwise, all functions that expect a byte position as a
   parameter assume that the given position is either the start of a byte
   sequence or one plus the length of the subject string. As in the string
   library, negative indices count from the end of the string.

   Functions that create byte sequences accept all values up to 0x7FFFFFFF,
   as defined in the original UTF-8 specification; that implies byte
   sequences of up to six bytes.

   Functions that interpret byte sequences only accept valid sequences (well
   formed and not overlong). By default, they only accept byte sequences that
   result in valid Unicode code points, rejecting values greater than 10FFFF
   and surrogates. A boolean argument lax, when available, lifts these
   checks, so that all values up to 0x7FFFFFFF are accepted. (Not well formed
   and overlong sequences are still rejected.)

     ----------------------------------------------------------------------

  utf8.char (···)

   Receives zero or more integers, converts each one to its corresponding
   UTF-8 byte sequence and returns a string with the concatenation of all
   these sequences.

     ----------------------------------------------------------------------

  utf8.charpattern

   The pattern (a string, not a function) "[\0-\x7F\xC2-\xFD][\x80-\xBF]*"
   (see §6.4.1), which matches exactly one UTF-8 byte sequence, assuming that
   the subject is a valid UTF-8 string.

     ----------------------------------------------------------------------

  utf8.codes (s [, lax])

   Returns values so that the construction

      for p, c in utf8.codes(s) do body end

   will iterate over all UTF-8 characters in string s, with p being the
   position (in bytes) and c the code point of each character. It raises an
   error if it meets any invalid byte sequence.

     ----------------------------------------------------------------------

  utf8.codepoint (s [, i [, j [, lax]]])

   Returns the code points (as integers) from all characters in s that start
   between byte position i and j (both included). The default for i is 1 and
   for j is i. It raises an error if it meets any invalid byte sequence.

     ----------------------------------------------------------------------

  utf8.len (s [, i [, j [, lax]]])

   Returns the number of UTF-8 characters in string s that start between
   positions i and j (both inclusive). The default for i is 1 and for j is
   -1. If it finds any invalid byte sequence, returns fail plus the position
   of the first invalid byte.

     ----------------------------------------------------------------------

  utf8.offset (s, n [, i])

   Returns the position (in bytes) where the encoding of the n-th character
   of s (counting from position i) starts. A negative n gets characters
   before position i. The default for i is 1 when n is non-negative and #s +
   1 otherwise, so that utf8.offset(s, -n) gets the offset of the n-th
   character from the end of the string. If the specified character is
   neither in the subject nor right after its end, the function returns fail.

   As a special case, when n is 0 the function returns the start of the
   encoding of the character that contains the i-th byte of s.

   This function assumes that s is a valid UTF-8 string.

6.6 – Table Manipulation

   This library provides generic functions for table manipulation. It
   provides all its functions inside the table table.

   Remember that, whenever an operation needs the length of a table, all
   caveats about the length operator apply (see §3.4.7). All functions ignore
   non-numeric keys in the tables given as arguments.

     ----------------------------------------------------------------------

  table.concat (list [, sep [, i [, j]]])

   Given a list where all elements are strings or numbers, returns the string
   list[i]..sep..list[i+1] ··· sep..list[j]. The default value for sep is the
   empty string, the default for i is 1, and the default for j is #list. If i
   is greater than j, returns the empty string.

     ----------------------------------------------------------------------

  table.insert (list, [pos,] value)

   Inserts element value at position pos in list, shifting up the elements
   list[pos], list[pos+1], ···, list[#list]. The default value for pos is
   #list+1, so that a call table.insert(t,x) inserts x at the end of the list
   t.

     ----------------------------------------------------------------------

  table.move (a1, f, e, t [,a2])

   Moves elements from the table a1 to the table a2, performing the
   equivalent to the following multiple assignment: a2[t],··· =
   a1[f],···,a1[e]. The default for a2 is a1. The destination range can
   overlap with the source range. The number of elements to be moved must fit
   in a Lua integer.

   Returns the destination table a2.

     ----------------------------------------------------------------------

  table.pack (···)

   Returns a new table with all arguments stored into keys 1, 2, etc. and
   with a field "n" with the total number of arguments. Note that the
   resulting table may not be a sequence, if some arguments are nil.

     ----------------------------------------------------------------------

  table.remove (list [, pos])

   Removes from list the element at position pos, returning the value of the
   removed element. When pos is an integer between 1 and #list, it shifts
   down the elements list[pos+1], list[pos+2], ···, list[#list] and erases
   element list[#list]; The index pos can also be 0 when #list is 0, or #list
   + 1.

   The default value for pos is #list, so that a call table.remove(l) removes
   the last element of the list l.

     ----------------------------------------------------------------------

  table.sort (list [, comp])

   Sorts the list elements in a given order, in-place, from list[1] to
   list[#list]. If comp is given, then it must be a function that receives
   two list elements and returns true when the first element must come before
   the second in the final order, so that, after the sort, i <= j implies not
   comp(list[j],list[i]). If comp is not given, then the standard Lua
   operator < is used instead.

   The comp function must define a consistent order; more formally, the
   function must define a strict weak order. (A weak order is similar to a
   total order, but it can equate different elements for comparison
   purposes.)

   The sort algorithm is not stable: Different elements considered equal by
   the given order may have their relative positions changed by the sort.

     ----------------------------------------------------------------------

  table.unpack (list [, i [, j]])

   Returns the elements from the given list. This function is equivalent to

      return list[i], list[i+1], ···, list[j]

   By default, i is 1 and j is #list.

6.7 – Mathematical Functions

   This library provides basic mathematical functions. It provides all its
   functions and constants inside the table math. Functions with the
   annotation "integer/float" give integer results for integer arguments and
   float results for non-integer arguments. The rounding functions math.ceil,
   math.floor, and math.modf return an integer when the result fits in the
   range of an integer, or a float otherwise.

     ----------------------------------------------------------------------

  math.abs (x)

   Returns the maximum value between x and -x. (integer/float)

     ----------------------------------------------------------------------

  math.acos (x)

   Returns the arc cosine of x (in radians).

     ----------------------------------------------------------------------

  math.asin (x)

   Returns the arc sine of x (in radians).

     ----------------------------------------------------------------------

  math.atan (y [, x])

   Returns the arc tangent of y/x (in radians), but uses the signs of both
   arguments to find the quadrant of the result. It also handles correctly
   the case of x being zero.

   The default value for x is 1, so that the call math.atan(y) returns the
   arc tangent of y.

     ----------------------------------------------------------------------

  math.ceil (x)

   Returns the smallest integral value greater than or equal to x.

     ----------------------------------------------------------------------

  math.cos (x)

   Returns the cosine of x (assumed to be in radians).

     ----------------------------------------------------------------------

  math.deg (x)

   Converts the angle x from radians to degrees.

     ----------------------------------------------------------------------

  math.exp (x)

   Returns the value e^x (where e is the base of natural logarithms).

     ----------------------------------------------------------------------

  math.floor (x)

   Returns the largest integral value less than or equal to x.

     ----------------------------------------------------------------------

  math.fmod (x, y)

   Returns the remainder of the division of x by y that rounds the quotient
   towards zero. (integer/float)

     ----------------------------------------------------------------------

  math.huge

   The float value HUGE_VAL, a value greater than any other numeric value.

     ----------------------------------------------------------------------

  math.log (x [, base])

   Returns the logarithm of x in the given base. The default for base is e
   (so that the function returns the natural logarithm of x).

     ----------------------------------------------------------------------

  math.max (x, ···)

   Returns the argument with the maximum value, according to the Lua operator
   <.

     ----------------------------------------------------------------------

  math.maxinteger

   An integer with the maximum value for an integer.

     ----------------------------------------------------------------------

  math.min (x, ···)

   Returns the argument with the minimum value, according to the Lua operator
   <.

     ----------------------------------------------------------------------

  math.mininteger

   An integer with the minimum value for an integer.

     ----------------------------------------------------------------------

  math.modf (x)

   Returns the integral part of x and the fractional part of x. Its second
   result is always a float.

     ----------------------------------------------------------------------

  math.pi

   The value of π.

     ----------------------------------------------------------------------

  math.rad (x)

   Converts the angle x from degrees to radians.

     ----------------------------------------------------------------------

  math.random ([m [, n]])

   When called without arguments, returns a pseudo-random float with uniform
   distribution in the range [0,1). When called with two integers m and n,
   math.random returns a pseudo-random integer with uniform distribution in
   the range [m, n]. The call math.random(n), for a positive n, is equivalent
   to math.random(1,n). The call math.random(0) produces an integer with all
   bits (pseudo)random.

   This function uses the xoshiro256** algorithm to produce pseudo-random
   64-bit integers, which are the results of calls with argument 0. Other
   results (ranges and floats) are unbiased extracted from these integers.

   Lua initializes its pseudo-random generator with the equivalent of a call
   to math.randomseed with no arguments, so that math.random should generate
   different sequences of results each time the program runs.

     ----------------------------------------------------------------------

  math.randomseed ([x [, y]])

   When called with at least one argument, the integer parameters x and y are
   joined into a 128-bit seed that is used to reinitialize the pseudo-random
   generator; equal seeds produce equal sequences of numbers. The default for
   y is zero.

   When called with no arguments, Lua generates a seed with a weak attempt
   for randomness.

   This function returns the two seed components that were effectively used,
   so that setting them again repeats the sequence.

   To ensure a required level of randomness to the initial state (or
   contrarily, to have a deterministic sequence, for instance when debugging
   a program), you should call math.randomseed with explicit arguments.

     ----------------------------------------------------------------------

  math.sin (x)

   Returns the sine of x (assumed to be in radians).

     ----------------------------------------------------------------------

  math.sqrt (x)

   Returns the square root of x. (You can also use the expression x^0.5 to
   compute this value.)

     ----------------------------------------------------------------------

  math.tan (x)

   Returns the tangent of x (assumed to be in radians).

     ----------------------------------------------------------------------

  math.tointeger (x)

   If the value x is convertible to an integer, returns that integer.
   Otherwise, returns fail.

     ----------------------------------------------------------------------

  math.type (x)

   Returns "integer" if x is an integer, "float" if it is a float, or fail if
   x is not a number.

     ----------------------------------------------------------------------

  math.ult (m, n)

   Returns a boolean, true if and only if integer m is below integer n when
   they are compared as unsigned integers.

6.8 – Input and Output Facilities

   The I/O library provides two different styles for file manipulation. The
   first one uses implicit file handles; that is, there are operations to set
   a default input file and a default output file, and all input/output
   operations are done over these default files. The second style uses
   explicit file handles.

   When using implicit file handles, all operations are supplied by table io.
   When using explicit file handles, the operation io.open returns a file
   handle and then all operations are supplied as methods of the file handle.

   The metatable for file handles provides metamethods for __gc and __close
   that try to close the file when called.

   The table io also provides three predefined file handles with their usual
   meanings from C: io.stdin, io.stdout, and io.stderr. The I/O library never
   closes these files.

   Unless otherwise stated, all I/O functions return fail on failure, plus an
   error message as a second result and a system-dependent error code as a
   third result, and some non-false value on success. On non-POSIX systems,
   the computation of the error message and error code in case of errors may
   be not thread safe, because they rely on the global C variable errno.

     ----------------------------------------------------------------------

  io.close ([file])

   Equivalent to file:close(). Without a file, closes the default output
   file.

     ----------------------------------------------------------------------

  io.flush ()

   Equivalent to io.output():flush().

     ----------------------------------------------------------------------

  io.input ([file])

   When called with a file name, it opens the named file (in text mode), and
   sets its handle as the default input file. When called with a file handle,
   it simply sets this file handle as the default input file. When called
   without arguments, it returns the current default input file.

   In case of errors this function raises the error, instead of returning an
   error code.

     ----------------------------------------------------------------------

  io.lines ([filename, ···])

   Opens the given file name in read mode and returns an iterator function
   that works like file:lines(···) over the opened file. When the iterator
   function fails to read any value, it automatically closes the file.
   Besides the iterator function, io.lines returns three other values: two
   nil values as placeholders, plus the created file handle. Therefore, when
   used in a generic for loop, the file is closed also if the loop is
   interrupted by an error or a break.

   The call io.lines() (with no file name) is equivalent to
   io.input():lines("l"); that is, it iterates over the lines of the default
   input file. In this case, the iterator does not close the file when the
   loop ends.

   In case of errors opening the file, this function raises the error,
   instead of returning an error code.

     ----------------------------------------------------------------------

  io.open (filename [, mode])

   This function opens a file, in the mode specified in the string mode. In
   case of success, it returns a new file handle.

   The mode string can be any of the following:
     * "r": read mode (the default);
     * "w": write mode;
     * "a": append mode;
     * "r+": update mode, all previous data is preserved;
     * "w+": update mode, all previous data is erased;
     * "a+": append update mode, previous data is preserved, writing is only
       allowed at the end of file.

   The mode string can also have a 'b' at the end, which is needed in some
   systems to open the file in binary mode.

     ----------------------------------------------------------------------

  io.output ([file])

   Similar to io.input, but operates over the default output file.

     ----------------------------------------------------------------------

  io.popen (prog [, mode])

   This function is system dependent and is not available on all platforms.

   Starts the program prog in a separated process and returns a file handle
   that you can use to read data from this program (if mode is "r", the
   default) or to write data to this program (if mode is "w").

     ----------------------------------------------------------------------

  io.read (···)

   Equivalent to io.input():read(···).

     ----------------------------------------------------------------------

  io.tmpfile ()

   In case of success, returns a handle for a temporary file. This file is
   opened in update mode and it is automatically removed when the program
   ends.

     ----------------------------------------------------------------------

  io.type (obj)

   Checks whether obj is a valid file handle. Returns the string "file" if
   obj is an open file handle, "closed file" if obj is a closed file handle,
   or fail if obj is not a file handle.

     ----------------------------------------------------------------------

  io.write (···)

   Equivalent to io.output():write(···).

     ----------------------------------------------------------------------

  file:close ()

   Closes file. Note that files are automatically closed when their handles
   are garbage collected, but that takes an unpredictable amount of time to
   happen.

   When closing a file handle created with io.popen, file:close returns the
   same values returned by os.execute.

     ----------------------------------------------------------------------

  file:flush ()

   Saves any written data to file.

     ----------------------------------------------------------------------

  file:lines (···)

   Returns an iterator function that, each time it is called, reads the file
   according to the given formats. When no format is given, uses "l" as a
   default. As an example, the construction

      for c in file:lines(1) do body end

   will iterate over all characters of the file, starting at the current
   position. Unlike io.lines, this function does not close the file when the
   loop ends.

     ----------------------------------------------------------------------

  file:read (···)

   Reads the file file, according to the given formats, which specify what to
   read. For each format, the function returns a string or a number with the
   characters read, or fail if it cannot read data with the specified format.
   (In this latter case, the function does not read subsequent formats.) When
   called without arguments, it uses a default format that reads the next
   line (see below).

   The available formats are
     * "n": reads a numeral and returns it as a float or an integer,
       following the lexical conventions of Lua. (The numeral may have
       leading whitespaces and a sign.) This format always reads the longest
       input sequence that is a valid prefix for a numeral; if that prefix
       does not form a valid numeral (e.g., an empty string, "0x", or
       "3.4e-") or it is too long (more than 200 characters), it is discarded
       and the format returns fail.
     * "a": reads the whole file, starting at the current position. On end of
       file, it returns the empty string; this format never fails.
     * "l": reads the next line skipping the end of line, returning fail on
       end of file. This is the default format.
     * "L": reads the next line keeping the end-of-line character (if
       present), returning fail on end of file.
     * number: reads a string with up to this number of bytes, returning fail
       on end of file. If number is zero, it reads nothing and returns an
       empty string, or fail on end of file.

   The formats "l" and "L" should be used only for text files.

     ----------------------------------------------------------------------

  file:seek ([whence [, offset]])

   Sets and gets the file position, measured from the beginning of the file,
   to the position given by offset plus a base specified by the string
   whence, as follows:
     * "set": base is position 0 (beginning of the file);
     * "cur": base is current position;
     * "end": base is end of file;

   In case of success, seek returns the final file position, measured in
   bytes from the beginning of the file. If seek fails, it returns fail, plus
   a string describing the error.

   The default value for whence is "cur", and for offset is 0. Therefore, the
   call file:seek() returns the current file position, without changing it;
   the call file:seek("set") sets the position to the beginning of the file
   (and returns 0); and the call file:seek("end") sets the position to the
   end of the file, and returns its size.

     ----------------------------------------------------------------------

  file:setvbuf (mode [, size])

   Sets the buffering mode for a file. There are three available modes:
     * "no": no buffering.
     * "full": full buffering.
     * "line": line buffering.

   For the last two cases, size is a hint for the size of the buffer, in
   bytes. The default is an appropriate size.

   The specific behavior of each mode is non portable; check the underlying
   ISO C function setvbuf in your platform for more details.

     ----------------------------------------------------------------------

  file:write (···)

   Writes the value of each of its arguments to file. The arguments must be
   strings or numbers.

   In case of success, this function returns file.

6.9 – Operating System Facilities

   This library is implemented through table os.

     ----------------------------------------------------------------------

  os.clock ()

   Returns an approximation of the amount in seconds of CPU time used by the
   program, as returned by the underlying ISO C function clock.

     ----------------------------------------------------------------------

  os.date ([format [, time]])

   Returns a string or a table containing date and time, formatted according
   to the given string format.

   If the time argument is present, this is the time to be formatted (see the
   os.time function for a description of this value). Otherwise, date formats
   the current time.

   If format starts with '!', then the date is formatted in Coordinated
   Universal Time. After this optional character, if format is the string
   "*t", then date returns a table with the following fields: year, month
   (1–12), day (1–31), hour (0–23), min (0–59), sec (0–61, due to leap
   seconds), wday (weekday, 1–7, Sunday is 1), yday (day of the year, 1–366),
   and isdst (daylight saving flag, a boolean). This last field may be absent
   if the information is not available.

   If format is not "*t", then date returns the date as a string, formatted
   according to the same rules as the ISO C function strftime.

   If format is absent, it defaults to "%c", which gives a human-readable
   date and time representation using the current locale.

   On non-POSIX systems, this function may be not thread safe because of its
   reliance on C function gmtime and C function localtime.

     ----------------------------------------------------------------------

  os.difftime (t2, t1)

   Returns the difference, in seconds, from time t1 to time t2 (where the
   times are values returned by os.time). In POSIX, Windows, and some other
   systems, this value is exactly t2-t1.

     ----------------------------------------------------------------------

  os.execute ([command])

   This function is equivalent to the ISO C function system. It passes
   command to be executed by an operating system shell. Its first result is
   true if the command terminated successfully, or fail otherwise. After this
   first result the function returns a string plus a number, as follows:
     * "exit": the command terminated normally; the following number is the
       exit status of the command.
     * "signal": the command was terminated by a signal; the following number
       is the signal that terminated the command.

   When called without a command, os.execute returns a boolean that is true
   if a shell is available.

     ----------------------------------------------------------------------

  os.exit ([code [, close]])

   Calls the ISO C function exit to terminate the host program. If code is
   true, the returned status is EXIT_SUCCESS; if code is false, the returned
   status is EXIT_FAILURE; if code is a number, the returned status is this
   number. The default value for code is true.

   If the optional second argument close is true, closes the Lua state before
   exiting.

     ----------------------------------------------------------------------

  os.getenv (varname)

   Returns the value of the process environment variable varname or fail if
   the variable is not defined.

     ----------------------------------------------------------------------

  os.remove (filename)

   Deletes the file (or empty directory, on POSIX systems) with the given
   name. If this function fails, it returns fail plus a string describing the
   error and the error code. Otherwise, it returns true.

     ----------------------------------------------------------------------

  os.rename (oldname, newname)

   Renames the file or directory named oldname to newname. If this function
   fails, it returns fail, plus a string describing the error and the error
   code. Otherwise, it returns true.

     ----------------------------------------------------------------------

  os.setlocale (locale [, category])

   Sets the current locale of the program. locale is a system-dependent
   string specifying a locale; category is an optional string describing
   which category to change: "all", "collate", "ctype", "monetary",
   "numeric", or "time"; the default category is "all". The function returns
   the name of the new locale, or fail if the request cannot be honored.

   If locale is the empty string, the current locale is set to an
   implementation-defined native locale. If locale is the string "C", the
   current locale is set to the standard C locale.

   When called with nil as the first argument, this function only returns the
   name of the current locale for the given category.

   This function may be not thread safe because of its reliance on C function
   setlocale.

     ----------------------------------------------------------------------

  os.time ([table])

   Returns the current time when called without arguments, or a time
   representing the local date and time specified by the given table. This
   table must have fields year, month, and day, and may have fields hour
   (default is 12), min (default is 0), sec (default is 0), and isdst
   (default is nil). Other fields are ignored. For a description of these
   fields, see the os.date function.

   When the function is called, the values in these fields do not need to be
   inside their valid ranges. For instance, if sec is -10, it means 10
   seconds before the time specified by the other fields; if hour is 1000, it
   means 1000 hours after the time specified by the other fields.

   The returned value is a number, whose meaning depends on your system. In
   POSIX, Windows, and some other systems, this number counts the number of
   seconds since some given start time (the "epoch"). In other systems, the
   meaning is not specified, and the number returned by time can be used only
   as an argument to os.date and os.difftime.

   When called with a table, os.time also normalizes all the fields
   documented in the os.date function, so that they represent the same time
   as before the call but with values inside their valid ranges.

     ----------------------------------------------------------------------

  os.tmpname ()

   Returns a string with a file name that can be used for a temporary file.
   The file must be explicitly opened before its use and explicitly removed
   when no longer needed.

   In POSIX systems, this function also creates a file with that name, to
   avoid security risks. (Someone else might create the file with wrong
   permissions in the time between getting the name and creating the file.)
   You still have to open the file to use it and to remove it (even if you do
   not use it).

   When possible, you may prefer to use io.tmpfile, which automatically
   removes the file when the program ends.

6.10 – The Debug Library

   This library provides the functionality of the debug interface (§4.7) to
   Lua programs. You should exert care when using this library. Several of
   its functions violate basic assumptions about Lua code (e.g., that
   variables local to a function cannot be accessed from outside; that
   userdata metatables cannot be changed by Lua code; that Lua programs do
   not crash) and therefore can compromise otherwise secure code. Moreover,
   some functions in this library may be slow.

   All functions in this library are provided inside the debug table. All
   functions that operate over a thread have an optional first argument which
   is the thread to operate over. The default is always the current thread.

     ----------------------------------------------------------------------

  debug.debug ()

   Enters an interactive mode with the user, running each string that the
   user enters. Using simple commands and other debug facilities, the user
   can inspect global and local variables, change their values, evaluate
   expressions, and so on. A line containing only the word cont finishes this
   function, so that the caller continues its execution.

   Note that commands for debug.debug are not lexically nested within any
   function and so have no direct access to local variables.

     ----------------------------------------------------------------------

  debug.gethook ([thread])

   Returns the current hook settings of the thread, as three values: the
   current hook function, the current hook mask, and the current hook count,
   as set by the debug.sethook function.

   Returns fail if there is no active hook.

     ----------------------------------------------------------------------

  debug.getinfo ([thread,] f [, what])

   Returns a table with information about a function. You can give the
   function directly or you can give a number as the value of f, which means
   the function running at level f of the call stack of the given thread:
   level 0 is the current function (getinfo itself); level 1 is the function
   that called getinfo (except for tail calls, which do not count in the
   stack); and so on. If f is a number greater than the number of active
   functions, then getinfo returns fail.

   The returned table can contain all the fields returned by lua_getinfo,
   with the string what describing which fields to fill in. The default for
   what is to get all information available, except the table of valid lines.
   If present, the option 'f' adds a field named func with the function
   itself. If present, the option 'L' adds a field named activelines with the
   table of valid lines.

   For instance, the expression debug.getinfo(1,"n").name returns a name for
   the current function, if a reasonable name can be found, and the
   expression debug.getinfo(print) returns a table with all available
   information about the print function.

     ----------------------------------------------------------------------

  debug.getlocal ([thread,] f, local)

   This function returns the name and the value of the local variable with
   index local of the function at level f of the stack. This function
   accesses not only explicit local variables, but also parameters and
   temporary values.

   The first parameter or local variable has index 1, and so on, following
   the order that they are declared in the code, counting only the variables
   that are active in the current scope of the function. Compile-time
   constants may not appear in this listing, if they were optimized away by
   the compiler. Negative indices refer to vararg arguments; -1 is the first
   vararg argument. The function returns fail if there is no variable with
   the given index, and raises an error when called with a level out of
   range. (You can call debug.getinfo to check whether the level is valid.)

   Variable names starting with '(' (open parenthesis) represent variables
   with no known names (internal variables such as loop control variables,
   and variables from chunks saved without debug information).

   The parameter f may also be a function. In that case, getlocal returns
   only the name of function parameters.

     ----------------------------------------------------------------------

  debug.getmetatable (value)

   Returns the metatable of the given value or nil if it does not have a
   metatable.

     ----------------------------------------------------------------------

  debug.getregistry ()

   Returns the registry table (see §4.3).

     ----------------------------------------------------------------------

  debug.getupvalue (f, up)

   This function returns the name and the value of the upvalue with index up
   of the function f. The function returns fail if there is no upvalue with
   the given index.

   (For Lua functions, upvalues are the external local variables that the
   function uses, and that are consequently included in its closure.)

   For C functions, this function uses the empty string "" as a name for all
   upvalues.

   Variable name '?' (interrogation mark) represents variables with no known
   names (variables from chunks saved without debug information).

     ----------------------------------------------------------------------

  debug.getuservalue (u, n)

   Returns the n-th user value associated to the userdata u plus a boolean,
   false if the userdata does not have that value.

     ----------------------------------------------------------------------

  debug.sethook ([thread,] hook, mask [, count])

   Sets the given function as the debug hook. The string mask and the number
   count describe when the hook will be called. The string mask may have any
   combination of the following characters, with the given meaning:
     * 'c': the hook is called every time Lua calls a function;
     * 'r': the hook is called every time Lua returns from a function;
     * 'l': the hook is called every time Lua enters a new line of code.

   Moreover, with a count different from zero, the hook is called also after
   every count instructions.

   When called without arguments, debug.sethook turns off the hook.

   When the hook is called, its first parameter is a string describing the
   event that has triggered its call: "call", "tail call", "return", "line",
   and "count". For line events, the hook also gets the new line number as
   its second parameter. Inside a hook, you can call getinfo with level 2 to
   get more information about the running function. (Level 0 is the getinfo
   function, and level 1 is the hook function.)

     ----------------------------------------------------------------------

  debug.setlocal ([thread,] level, local, value)

   This function assigns the value value to the local variable with index
   local of the function at level level of the stack. The function returns
   fail if there is no local variable with the given index, and raises an
   error when called with a level out of range. (You can call getinfo to
   check whether the level is valid.) Otherwise, it returns the name of the
   local variable.

   See debug.getlocal for more information about variable indices and names.

     ----------------------------------------------------------------------

  debug.setmetatable (value, table)

   Sets the metatable for the given value to the given table (which can be
   nil). Returns value.

     ----------------------------------------------------------------------

  debug.setupvalue (f, up, value)

   This function assigns the value value to the upvalue with index up of the
   function f. The function returns fail if there is no upvalue with the
   given index. Otherwise, it returns the name of the upvalue.

   See debug.getupvalue for more information about upvalues.

     ----------------------------------------------------------------------

  debug.setuservalue (udata, value, n)

   Sets the given value as the n-th user value associated to the given udata.
   udata must be a full userdata.

   Returns udata, or fail if the userdata does not have that value.

     ----------------------------------------------------------------------

  debug.traceback ([thread,] [message [, level]])

   If message is present but is neither a string nor nil, this function
   returns message without further processing. Otherwise, it returns a string
   with a traceback of the call stack. The optional message string is
   appended at the beginning of the traceback. An optional level number tells
   at which level to start the traceback (default is 1, the function calling
   traceback).

     ----------------------------------------------------------------------

  debug.upvalueid (f, n)

   Returns a unique identifier (as a light userdata) for the upvalue numbered
   n from the given function.

   These unique identifiers allow a program to check whether different
   closures share upvalues. Lua closures that share an upvalue (that is, that
   access a same external local variable) will return identical ids for those
   upvalue indices.

     ----------------------------------------------------------------------

  debug.upvaluejoin (f1, n1, f2, n2)

   Make the n1-th upvalue of the Lua closure f1 refer to the n2-th upvalue of
   the Lua closure f2.

                               7 – Lua Standalone

   Although Lua has been designed as an extension language, to be embedded in
   a host C program, it is also frequently used as a standalone language. An
   interpreter for Lua as a standalone language, called simply lua, is
   provided with the standard distribution. The standalone interpreter
   includes all standard libraries. Its usage is:

      lua [options] [script [args]]

   The options are:
     * -e stat: execute string stat;
     * -i: enter interactive mode after running script;
     * -l mod: "require" mod and assign the result to global mod;
     * -v: print version information;
     * -E: ignore environment variables;
     * -W: turn warnings on;
     * --: stop handling options;
     * -: execute stdin as a file and stop handling options.

   After handling its options, lua runs the given script. When called without
   arguments, lua behaves as lua -v -i when the standard input (stdin) is a
   terminal, and as lua - otherwise.

   When called without the option -E, the interpreter checks for an
   environment variable LUA_INIT_5_4 (or LUA_INIT if the versioned name is
   not defined) before running any argument. If the variable content has the
   format @filename, then lua executes the file. Otherwise, lua executes the
   string itself.

   When called with the option -E, Lua does not consult any environment
   variables. In particular, the values of package.path and package.cpath are
   set with the default paths defined in luaconf.h.

   The options -e, -l, and -W are handled in the order they appear. For
   instance, an invocation like

      $ lua -e 'a=1' -llib1 script.lua

   will first set a to 1, then require the library lib1, and finally run the
   file script.lua with no arguments. (Here $ is the shell prompt. Your
   prompt may be different.)

   Before running any code, lua collects all command-line arguments in a
   global table called arg. The script name goes to index 0, the first
   argument after the script name goes to index 1, and so on. Any arguments
   before the script name (that is, the interpreter name plus its options) go
   to negative indices. For instance, in the call

      $ lua -la b.lua t1 t2

   the table is like this:

      arg = { [-2] = "lua", [-1] = "-la",
              [0] = "b.lua",
              [1] = "t1", [2] = "t2" }

   If there is no script in the call, the interpreter name goes to index 0,
   followed by the other arguments. For instance, the call

      $ lua -e "print(arg[1])"

   will print "-e". If there is a script, the script is called with arguments
   arg[1], ···, arg[#arg]. Like all chunks in Lua, the script is compiled as
   a vararg function.

   In interactive mode, Lua repeatedly prompts and waits for a line. After
   reading a line, Lua first try to interpret the line as an expression. If
   it succeeds, it prints its value. Otherwise, it interprets the line as a
   statement. If you write an incomplete statement, the interpreter waits for
   its completion by issuing a different prompt.

   If the global variable _PROMPT contains a string, then its value is used
   as the prompt. Similarly, if the global variable _PROMPT2 contains a
   string, its value is used as the secondary prompt (issued during
   incomplete statements).

   In case of unprotected errors in the script, the interpreter reports the
   error to the standard error stream. If the error object is not a string
   but has a metamethod __tostring, the interpreter calls this metamethod to
   produce the final message. Otherwise, the interpreter converts the error
   object to a string and adds a stack traceback to it. When warnings are on,
   they are simply printed in the standard error output.

   When finishing normally, the interpreter closes its main Lua state (see
   lua_close). The script can avoid this step by calling os.exit to
   terminate.

   To allow the use of Lua as a script interpreter in Unix systems, Lua skips
   the first line of a file chunk if it starts with #. Therefore, Lua scripts
   can be made into executable programs by using chmod +x and the #! form, as
   in

      #!/usr/local/bin/lua

   Of course, the location of the Lua interpreter may be different in your
   machine. If lua is in your PATH, then

      #!/usr/bin/env lua

   is a more portable solution.

                8 – Incompatibilities with the Previous Version

   Here we list the incompatibilities that you may find when moving a program
   from Lua 5.3 to Lua 5.4.

   You can avoid some incompatibilities by compiling Lua with appropriate
   options (see file luaconf.h). However, all these compatibility options
   will be removed in the future. More often than not, compatibility issues
   arise when these compatibility options are removed. So, whenever you have
   the chance, you should try to test your code with a version of Lua
   compiled with all compatibility options turned off. That will ease
   transitions to newer versions of Lua.

   Lua versions can always change the C API in ways that do not imply
   source-code changes in a program, such as the numeric values for constants
   or the implementation of functions as macros. Therefore, you should never
   assume that binaries are compatible between different Lua versions. Always
   recompile clients of the Lua API when using a new version.

   Similarly, Lua versions can always change the internal representation of
   precompiled chunks; precompiled chunks are not compatible between
   different Lua versions.

   The standard paths in the official distribution may change between
   versions.

8.1 – Incompatibilities in the Language

     * The coercion of strings to numbers in arithmetic and bitwise
       operations has been removed from the core language. The string library
       does a similar job for arithmetic (but not for bitwise) operations
       using the string metamethods. However, unlike in previous versions,
       the new implementation preserves the implicit type of the numeral in
       the string. For instance, the result of "1" + "2" now is an integer,
       not a float.
     * Literal decimal integer constants that overflow are read as floats,
       instead of wrapping around. You can use hexadecimal notation for such
       constants if you want the old behavior (reading them as integers with
       wrap around).
     * The use of the __lt metamethod to emulate __le has been removed. When
       needed, this metamethod must be explicitly defined.
     * The semantics of the numerical for loop over integers changed in some
       details. In particular, the control variable never wraps around.
     * A label for a goto cannot be declared where a label with the same name
       is visible, even if this other label is declared in an enclosing
       block.
     * When finalizing an object, Lua does not ignore __gc metamethods that
       are not functions. Any value will be called, if present. (Non-callable
       values will generate a warning, like any other error when calling a
       finalizer.)

8.2 – Incompatibilities in the Libraries

     * The function print does not call tostring to format its arguments;
       instead, it has this functionality hardwired. You should use
       __tostring to modify how values are printed.
     * The pseudo-random number generator used by the function math.random
       now starts with a somewhat random seed. Moreover, it uses a different
       algorithm.
     * By default, the decoding functions in the utf8 library do not accept
       surrogates as valid code points. An extra parameter in these functions
       makes them more permissive.
     * The options "setpause" and "setstepmul" of the function collectgarbage
       are deprecated. You should use the new option "incremental" to set
       them.
     * The function io.lines now returns four values, instead of just one.
       That can be a problem when it is used as the sole argument to another
       function that has optional parameters, such as in
       load(io.lines(filename, "L")). To fix that issue, you can wrap the
       call into parentheses, to adjust its number of results to one.

8.3 – Incompatibilities in the API

     * Full userdata now has an arbitrary number of associated user values.
       Therefore, the functions lua_newuserdata, lua_setuservalue, and
       lua_getuservalue were replaced by lua_newuserdatauv,
       lua_setiuservalue, and lua_getiuservalue, which have an extra
       argument.

       For compatibility, the old names still work as macros assuming one
       single user value. Note, however, that userdata with zero user values
       are more efficient memory-wise.
     * The function lua_resume has an extra parameter. This out parameter
       returns the number of values on the top of the stack that were yielded
       or returned by the coroutine. (In previous versions, those values were
       the entire stack.)
     * The function lua_version returns the version number, instead of an
       address of the version number. The Lua core should work correctly with
       libraries using their own static copies of the same core, so there is
       no need to check whether they are using the same address space.
     * The constant LUA_ERRGCMM was removed. Errors in finalizers are never
       propagated; instead, they generate a warning.
     * The options LUA_GCSETPAUSE and LUA_GCSETSTEPMUL of the function lua_gc
       are deprecated. You should use the new option LUA_GCINC to set them.

                         9 – The Complete Syntax of Lua

   Here is the complete syntax of Lua in extended BNF. As usual in extended
   BNF, {A} means 0 or more As, and [A] means an optional A. (For operator
   precedences, see §3.4.8; for a description of the terminals Name, Numeral,
   and LiteralString, see §3.1.)


         chunk ::= block

         block ::= {stat} [retstat]

         stat ::=  ‘;’ |
                  varlist ‘=’ explist |
                  functioncall |
                  label |
                  break |
                  goto Name |
                  do block end |
                  while exp do block end |
                  repeat block until exp |
                  if exp then block {elseif exp then block} [else block] end |
                  for Name ‘=’ exp ‘,’ exp [‘,’ exp] do block end |
                  for namelist in explist do block end |
                  function funcname funcbody |
                  local function Name funcbody |
                  local attnamelist [‘=’ explist]

         attnamelist ::=  Name attrib {‘,’ Name attrib}

         attrib ::= [‘<’ Name ‘>’]

         retstat ::= return [explist] [‘;’]

         label ::= ‘::’ Name ‘::’

         funcname ::= Name {‘.’ Name} [‘:’ Name]

         varlist ::= var {‘,’ var}

         var ::=  Name | prefixexp ‘[’ exp ‘]’ | prefixexp ‘.’ Name

         namelist ::= Name {‘,’ Name}

         explist ::= exp {‘,’ exp}

         exp ::=  nil | false | true | Numeral | LiteralString | ‘...’ | functiondef |
                  prefixexp | tableconstructor | exp binop exp | unop exp

         prefixexp ::= var | functioncall | ‘(’ exp ‘)’

         functioncall ::=  prefixexp args | prefixexp ‘:’ Name args

         args ::=  ‘(’ [explist] ‘)’ | tableconstructor | LiteralString

         functiondef ::= function funcbody

         funcbody ::= ‘(’ [parlist] ‘)’ block end

         parlist ::= namelist [‘,’ ‘...’] | ‘...’

         tableconstructor ::= ‘{’ [fieldlist] ‘}’

         fieldlist ::= field {fieldsep field} [fieldsep]

         field ::= ‘[’ exp ‘]’ ‘=’ exp | Name ‘=’ exp | exp

         fieldsep ::= ‘,’ | ‘;’

         binop ::=  ‘+’ | ‘-’ | ‘*’ | ‘/’ | ‘//’ | ‘^’ | ‘%’ |
                  ‘&’ | ‘~’ | ‘|’ | ‘>>’ | ‘<<’ | ‘..’ |
                  ‘<’ | ‘<=’ | ‘>’ | ‘>=’ | ‘==’ | ‘~=’ |
                  and | or

         unop ::= ‘-’ | not | ‘#’ | ‘~’


   Last update: Thu Jan 13 11:33:16 UTC 2022
