\input texinfo @c -*-texinfo-*-
@comment %**start of header
@setfilename bison.info
-@settitle Bison 1.24
+@include version.texi
+@settitle Bison @value{VERSION}
@setchapternewpage odd
@iftex
@c the smallbook format.
@c @smallbook
-@c next time, consider using @set for edition number, etc...
-
@c Set following if you have the new `shorttitlepage' command
@c @clear shorttitlepage-enabled
@c @set shorttitlepage-enabled
@end ifinfo
@comment %**end of header
+@ifinfo
+@format
+START-INFO-DIR-ENTRY
+* bison: (bison). GNU Project parser generator (yacc replacement).
+END-INFO-DIR-ENTRY
+@end format
+@end ifinfo
+
@ifinfo
This file documents the Bison parser generator.
-Copyright (C) 1988, 89, 90, 91, 92, 93, 1995 Free Software Foundation, Inc.
+Copyright (C) 1988, 89, 90, 91, 92, 93, 95, 98, 1999 Free Software Foundation, Inc.
Permission is granted to make and distribute verbatim copies of
this manual provided the copyright notice and this permission notice
@titlepage
@title Bison
@subtitle The YACC-compatible Parser Generator
-@subtitle May 1995, Bison Version 1.24
+@subtitle @value{UPDATED}, Bison Version @value{VERSION}
@author by Charles Donnelly and Richard Stallman
@page
@vskip 0pt plus 1filll
-Copyright @copyright{} 1988, 89, 90, 91, 92, 93, 1995 Free Software
+Copyright @copyright{} 1988, 89, 90, 91, 92, 93, 95, 98, 1999 Free Software
Foundation
@sp 2
Published by the Free Software Foundation @*
-675 Massachusetts Avenue @*
-Cambridge, MA 02139 USA @*
-Printed copies are available for $15 each.@*
-ISBN-1-882114-30-2
+59 Temple Place, Suite 330 @*
+Boston, MA 02111-1307 USA @*
+Printed copies are available from the Free Software Foundation.@*
+ISBN 1-882114-44-2
Permission is granted to make and distribute verbatim copies of
this manual provided the copyright notice and this permission notice
@node Top, Introduction, (dir), (dir)
@ifinfo
-This manual documents version 1.24 of Bison.
+This manual documents version @value{VERSION} of Bison.
@end ifinfo
@menu
don't know Bison or Yacc, start by reading these chapters. Reference
chapters follow which describe specific aspects of Bison in detail.
-Bison was written primarily by Robert Corbett; Richard Stallman made
-it Yacc-compatible. This edition corresponds to version 1.24 of Bison.
+Bison was written primarily by Robert Corbett; Richard Stallman made it
+Yacc-compatible. Wilfred Hansen of Carnegie Mellon University added
+multicharacter string literals and other features.
+
+This edition corresponds to version @value{VERSION} of Bison.
@node Conditions, Copying, Introduction, Top
@unnumbered Conditions for Using Bison
As of Bison version 1.24, we have changed the distribution terms for
-@code{yyparse} to permit using Bison's output in non-free programs.
+@code{yyparse} to permit using Bison's output in nonfree programs.
Formerly, Bison parsers could be used only in programs that were free
software.
The other GNU programming tools, such as the GNU C compiler, have never
-had such a requirement. They could always be used for non-free
+had such a requirement. They could always be used for nonfree
software. The reason Bison was different was not due to a special
policy decision; it resulted from applying the usual General Public
License to all of the Bison source code.
@display
Copyright @copyright{} 1989, 1991 Free Software Foundation, Inc.
-675 Mass Ave, Cambridge, MA 02139, USA
+59 Temple Place - Suite 330, Boston, MA 02111-1307, USA
Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.
You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
-Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
+Foundation, Inc., 59 Temple Place - Suite 330,
+Boston, MA 02111-1307, USA.
@end smallexample
Also add information on how to contact you by electronic and paper mail.
@code{RETURN}. A terminal symbol that stands for a particular keyword in
the language should be named after that keyword converted to upper case.
The terminal symbol @code{error} is reserved for error recovery.
-@xref{Symbols}.@refill
+@xref{Symbols}.
A terminal symbol can also be represented as a character literal, just like
a C character constant. You should do this whenever a token is just a
single character (parenthesis, plus-sign, etc.): use that same character in
a literal as the terminal symbol for that token.
+A third way to represent a terminal symbol is with a C string constant
+containing several characters. @xref{Symbols}, for more information.
+
The grammar rules also have an expression in Bison syntax. For example,
here is the Bison rule for a C @code{return} statement. The semicolon in
quotes is a literal character token, representing part of the C syntax for
(@pxref{Error Recovery}). Otherwise, @code{yyparse} returns nonzero. We
have not written any error rules in this example, so any invalid input will
cause the calculator program to exit. This is not clean behavior for a
-real calculator, but it is adequate in the first example.
+real calculator, but it is adequate for the first example.
@node Rpcalc Gen, Rpcalc Compile, Rpcalc Error, RPN Calc
@subsection Running Bison to Make the Parser
It is easy to add new operators to the infix calculator as long as they are
only single-character literals. The lexical analyzer @code{yylex} passes
-back all non-number characters as tokens, so new grammar rules suffice for
+back all nonnumber characters as tokens, so new grammar rules suffice for
adding a new operator. But we want something more flexible: built-in
functions whose syntax has this form:
Symbol names can contain letters, digits (not at the beginning),
underscores and periods. Periods make sense only in nonterminals.
-There are two ways of writing terminal symbols in the grammar:
+There are three ways of writing terminal symbols in the grammar:
@itemize @bullet
@item
@cindex character token
@cindex literal token
@cindex single-character literal
-A @dfn{character token type} (or @dfn{literal token}) is written in
-the grammar using the same syntax used in C for character constants;
-for example, @code{'+'} is a character token type. A character token
-type doesn't need to be declared unless you need to specify its
-semantic value data type (@pxref{Value Type, ,Data Types of Semantic Values}), associativity, or
-precedence (@pxref{Precedence, ,Operator Precedence}).
+A @dfn{character token type} (or @dfn{literal character token}) is
+written in the grammar using the same syntax used in C for character
+constants; for example, @code{'+'} is a character token type. A
+character token type doesn't need to be declared unless you need to
+specify its semantic value data type (@pxref{Value Type, ,Data Types of
+Semantic Values}), associativity, or precedence (@pxref{Precedence,
+,Operator Precedence}).
By convention, a character token type is used only to represent a
token that consists of that particular character. Thus, the token
All the usual escape sequences used in character literals in C can be
used in Bison as well, but you must not use the null character as a
-character literal because its ASCII code, zero, is the code
-@code{yylex} returns for end-of-input (@pxref{Calling Convention, ,Calling Convention for @code{yylex}}).
+character literal because its ASCII code, zero, is the code @code{yylex}
+returns for end-of-input (@pxref{Calling Convention, ,Calling Convention
+for @code{yylex}}).
+
+@item
+@cindex string token
+@cindex literal string token
+@cindex multicharacter literal
+A @dfn{literal string token} is written like a C string constant; for
+example, @code{"<="} is a literal string token. A literal string token
+doesn't need to be declared unless you need to specify its semantic
+value data type (@pxref{Value Type}), associativity, precedence
+(@pxref{Precedence}).
+
+You can associate the literal string token with a symbolic name as an
+alias, using the @code{%token} declaration (@pxref{Token Decl, ,Token
+Declarations}). If you don't do that, the lexical analyzer has to
+retrieve the token number for the literal string token from the
+@code{yytname} table (@pxref{Calling Convention}).
+
+@strong{WARNING}: literal string tokens do not work in Yacc.
+
+By convention, a literal string token is used only to represent a token
+that consists of that particular string. Thus, you should use the token
+type @code{"<="} to represent the string @samp{<=} as a token. Bison
+does not enforce this convention, but if you depart from it, people who
+read your program will be confused.
+
+All the escape sequences used in string literals in C can be used in
+Bison as well. A literal string token must contain two or more
+characters; for a token containing just one character, use a character
+token (see above).
@end itemize
How you choose to write a terminal symbol has no effect on its
@end example
@noindent
-where @var{result} is the nonterminal symbol that this rule describes
+where @var{result} is the nonterminal symbol that this rule describes,
and @var{components} are various terminal and nonterminal symbols that
are put together by this rule (@pxref{Symbols}).
A rule is called @dfn{recursive} when its @var{result} nonterminal appears
also on its right hand side. Nearly all Bison grammars need to use
recursion, because that is the only way to define a sequence of any number
-of somethings. Consider this recursive definition of a comma-separated
-sequence of one or more expressions:
+of a particular thing. Consider this recursive definition of a
+comma-separated sequence of one or more expressions:
@example
@group
@subsection Token Type Names
@cindex declaring token type names
@cindex token type names, declaring
+@cindex declaring literal string tokens
@findex %token
The basic way to declare a token type name (terminal symbol) is as follows:
can use the name @var{name} to stand for this token type's code.
Alternatively, you can use @code{%left}, @code{%right}, or @code{%nonassoc}
-instead of @code{%token}, if you wish to specify precedence.
+instead of @code{%token}, if you wish to specify associativity and precedence.
@xref{Precedence Decl, ,Operator Precedence}.
You can explicitly specify the numeric code for a token type by appending
@end group
@end example
+You can associate a literal string token with a token type name by
+writing the literal string at the end of a @code{%token}
+declaration which declares the name. For example:
+
+@example
+%token arrow "=>"
+@end example
+
+@noindent
+For example, a grammar for the C language might specify these names with
+equivalent literal string tokens:
+
+@example
+%token <operator> OR "||"
+%token <operator> LE 134 "<="
+%left OR "<="
+@end example
+
+@noindent
+Once you equate the literal string and the token name, you can use them
+interchangeably in further declarations or the grammar rules. The
+@code{yylex} function can use the token name or the literal string to
+obtain the token type code number (@pxref{Calling Convention}).
+
@node Precedence Decl, Union Decl, Token Decl, Declarations
@subsection Operator Precedence
@cindex precedence declarations
the same @code{%type} declaration, if they have the same value type. Use
spaces to separate the symbol names.
+You can also declare the value type of a terminal symbol. To do this,
+use the same @code{<@var{type}>} construction in a declaration for the
+terminal symbol. All kinds of token declarations allow
+@code{<@var{type}>}.
+
@node Expect Decl, Start Decl, Type Decl, Declarations
@subsection Suppressing Conflict Warnings
@cindex suppressing conflict warnings
handler. In systems with multiple threads of control, a nonreentrant
program must be called only within interlocks.
-The Bison parser is not normally a reentrant program, because it uses
-statically allocated variables for communication with @code{yylex}. These
-variables include @code{yylval} and @code{yylloc}.
+Normally, Bison generates a parser which is not reentrant. This is
+suitable for most uses, and it permits compatibility with YACC. (The
+standard YACC interfaces are inherently nonreentrant, because they use
+statically allocated variables for communication with @code{yylex},
+including @code{yylval} and @code{yylloc}.)
-The Bison declaration @code{%pure_parser} says that you want the parser
-to be reentrant. It looks like this:
+Alternatively, you can generate a pure, reentrant parser. The Bison
+declaration @code{%pure_parser} says that you want the parser to be
+reentrant. It looks like this:
@example
%pure_parser
@end example
-The effect is that the two communication variables become local
-variables in @code{yyparse}, and a different calling convention is used
-for the lexical analyzer function @code{yylex}. @xref{Pure Calling,
-,Calling Conventions for Pure Parsers}, for the details of this. The
-variable @code{yynerrs} also becomes local in @code{yyparse}
-(@pxref{Error Reporting, ,The Error Reporting Function @code{yyerror}}).
-The convention for calling @code{yyparse} itself is unchanged.
+The result is that the communication variables @code{yylval} and
+@code{yylloc} become local variables in @code{yyparse}, and a different
+calling convention is used for the lexical analyzer function
+@code{yylex}. @xref{Pure Calling, ,Calling Conventions for Pure
+Parsers}, for the details of this. The variable @code{yynerrs} also
+becomes local in @code{yyparse} (@pxref{Error Reporting, ,The Error
+Reporting Function @code{yyerror}}). The convention for calling
+@code{yyparse} itself is unchanged.
+
+Whether the parser is pure has nothing to do with the grammar rules.
+You can generate either a pure parser or a nonreentrant parser from any
+valid grammar.
@node Decl Summary, , Pure Decl, Declarations
@subsection Bison Declaration Summary
@item %pure_parser
Request a pure (reentrant) parser program (@pxref{Pure Decl, ,A Pure (Reentrant) Parser}).
+
+@item %no_lines
+Don't generate any @code{#line} preprocessor commands in the parser
+file. Ordinarily Bison writes these commands in the parser file so that
+the C compiler and debuggers will associate errors and object code with
+your source file (the grammar file). This directive causes them to
+associate errors with the parser file, treating it an independent source
+file in its own right.
+
+@item %raw
+The output file @file{@var{name}.h} normally defines the tokens with
+Yacc-compatible token numbers. If this option is specified, the
+internal Bison numbers are used instead. (Yacc-compatible numbers start
+at 257 except for single-character tokens; Bison assigns token numbers
+sequentially for all tokens starting at 3.)
+
+@item %token_table
+Generate an array of token names in the parser file. The name of the
+array is @code{yytname}; @code{yytname[@var{i}]} is the name of the
+token whose internal Bison token code number is @var{i}. The first three
+elements of @code{yytname} are always @code{"$"}, @code{"error"}, and
+@code{"$illegal"}; after these come the symbols defined in the grammar
+file.
+
+For single-character literal tokens and literal string tokens, the name
+in the table includes the single-quote or double-quote characters: for
+example, @code{"'+'"} is a single-character literal and @code{"\"<=\""}
+is a literal string token. All the characters of the literal string
+token appear verbatim in the string found in the table; even
+double-quote characters are not escaped. For example, if the token
+consists of three characters @samp{*"*}, its string in @code{yytname}
+contains @samp{"*"*"}. (In C, that would be written as
+@code{"\"*\"*\""}).
+
+When you specify @code{%token_table}, Bison also generates macro
+definitions for macros @code{YYNTOKENS}, @code{YYNNTS}, and
+@code{YYNRULES}, and @code{YYNSTATES}:
+
+@table @code
+@item YYNTOKENS
+The highest token number, plus one.
+@item YYNNTS
+The number of nonterminal symbols.
+@item YYNRULES
+The number of grammar rules,
+@item YYNSTATES
+The number of parser states (@pxref{Parser States}).
+@end table
@end table
-@node Multiple Parsers, , Declarations, Grammar File
+@node Multiple Parsers,, Declarations, Grammar File
@section Multiple Parsers in the Same Program
Most programs that use Bison parse only one language and therefore contain
This interface has been designed so that the output from the @code{lex}
utility can be used without change as the definition of @code{yylex}.
+If the grammar uses literal string tokens, there are two ways that
+@code{yylex} can determine the token type codes for them:
+
+@itemize @bullet
+@item
+If the grammar defines symbolic token names as aliases for the
+literal string tokens, @code{yylex} can use these symbolic names like
+all others. In this case, the use of the literal string tokens in
+the grammar file has no effect on @code{yylex}.
+
+@item
+@code{yylex} can find the multicharacter token in the @code{yytname}
+table. The index of the token in the table is the token type's code.
+The name of a multicharacter token is recorded in @code{yytname} with a
+double-quote, the token's characters, and another double-quote. The
+token's characters are not escaped in any way; they appear verbatim in
+the contents of the string in the table.
+
+Here's code for looking up a token in @code{yytname}, assuming that the
+characters of the token are stored in @code{token_buffer}.
+
+@smallexample
+for (i = 0; i < YYNTOKENS; i++)
+ @{
+ if (yytname[i] != 0
+ && yytname[i][0] == '"'
+ && strncmp (yytname[i] + 1, token_buffer,
+ strlen (token_buffer))
+ && yytname[i][strlen (token_buffer) + 1] == '"'
+ && yytname[i][strlen (token_buffer) + 2] == 0)
+ break;
+ @}
+@end smallexample
+
+The @code{yytname} table is generated only if you use the
+@code{%token_table} declaration. @xref{Decl Summary}.
+@end itemize
+
@node Token Values, Token Positions, Calling Convention, Lexical
@subsection Semantic Values of Tokens
only one argument.
@vindex YYPARSE_PARAM
-You can pass parameter information to a reentrant parser in a reentrant
-way. Define the macro @code{YYPARSE_PARAM} as a variable name. The
-resulting @code{yyparse} function then accepts one argument, of type
-@code{void *}, with that name.
+If you use a reentrant parser, you can optionally pass additional
+parameter information to it in a reentrant way. To do so, define the
+macro @code{YYPARSE_PARAM} as a variable name. This modifies the
+@code{yyparse} function to accept one argument, of type @code{void *},
+with that name.
When you call @code{yyparse}, pass the address of an object, casting the
address to @code{void *}. The grammar actions can refer to the contents
the proper object type, or you can declare it as @code{void *} and
access the contents as shown above.
+You can use @samp{%pure_parser} to request a reentrant parser without
+also using @code{YYPARSE_PARAM}. Then you should call @code{yyparse}
+with no arguments, as usual.
+
@node Error Reporting, Action Features, Lexical, Interface
@section The Error Reporting Function @code{yyerror}
@cindex error reporting function
@cindex syntax error
The Bison parser detects a @dfn{parse error} or @dfn{syntax error}
-whenever it reads a token which cannot satisfy any syntax rule. A
+whenever it reads a token which cannot satisfy any syntax rule. An
action in the grammar can also explicitly proclaim an error, using the
macro @code{YYERROR} (@pxref{Action Features, ,Special Features for Use in Actions}).
@};
@end example
-Thus, to get the starting line number of the third component, use
-@samp{@@3.first_line}.
+Thus, to get the starting line number of the third component, you would
+use @samp{@@3.first_line}.
In order for the members of this structure to contain valid information,
you must make @code{yylex} supply this information about each token.
@noindent
Suppose the parser has seen the tokens @samp{1}, @samp{-} and @samp{2};
-should it reduce them via the rule for the addition operator? It depends
+should it reduce them via the rule for the subtraction operator? It depends
on the next token. Of course, if the next token is @samp{)}, we must
reduce; shifting is invalid because no single rule can reduce the token
sequence @w{@samp{- 2 )}} or anything starting with that. But if the next
To decide which one Bison should do, we must consider the
results. If the next operator token @var{op} is shifted, then it
must be reduced first in order to permit another opportunity to
-reduce the sum. The result is (in effect) @w{@samp{1 - (2
+reduce the difference. The result is (in effect) @w{@samp{1 - (2
@var{op} 3)}}. On the other hand, if the subtraction is reduced
before shifting @var{op}, the result is @w{@samp{(1 - 2) @var{op}
3}}. Clearly, then, the choice of shift or reduce should depend
Unfortunately, the name being declared is separated from the declaration
construct itself by a complicated syntactic structure---the ``declarator''.
-As a result, the part of Bison parser for C needs to be duplicated, with
+As a result, part of the Bison parser for C needs to be duplicated, with
all the nonterminal names changed: once for parsing a declaration in which
a typedef name can be redefined, and once for parsing a declaration in
which that can't be done. Here is a part of the duplication, with actions
@menu
* Bison Options:: All the options described in detail,
in alphabetical order by short options.
+* Environment Variables:: Variables which affect Bison execution.
* Option Cross Key:: Alphabetical list of long options.
* VMS Invocation:: Bison command syntax on VMS.
@end menu
-@node Bison Options, Option Cross Key, , Invocation
+@node Bison Options, Environment Variables, , Invocation
@section Bison Options
Bison supports both traditional single-letter options and mnemonic long
Ordinarily Bison puts them in the parser file so that the C compiler
and debuggers will associate errors with your source file, the
grammar file. This option causes them to associate errors with the
-parser file, treating it an independent source file in its own right.
+parser file, treating it as an independent source file in its own right.
+
+@item -n
+@itemx --no-parser
+Do not include any C code in the parser file; generate tables only. The
+parser file contains just @code{#define} directives and static variable
+declarations.
+
+This option also tells Bison to write the C code for the grammar actions
+into a file named @file{@var{filename}.act}, in the form of a
+brace-surrounded body fit for a @code{switch} statement.
@item -o @var{outfile}
@itemx --output-file=@var{outfile}
Specify the name @var{outfile} for the parser file.
The other output files' names are constructed from @var{outfile}
-as described under the @samp{-v} and @samp{-d} switches.
+as described under the @samp{-v} and @samp{-d} options.
@item -p @var{prefix}
@itemx --name-prefix=@var{prefix}
@xref{Multiple Parsers, ,Multiple Parsers in the Same Program}.
+@item -r
+@itemx --raw
+Pretend that @code{%raw} was specified. @xref{Decl Summary}.
+
@item -t
@itemx --debug
Output a definition of the macro @code{YYDEBUG} into the parser file,
@itemx --fixed-output-files
Equivalent to @samp{-o y.tab.c}; the parser output file is called
@file{y.tab.c}, and the other outputs are called @file{y.output} and
-@file{y.tab.h}. The purpose of this switch is to imitate Yacc's output
+@file{y.tab.h}. The purpose of this option is to imitate Yacc's output
file name conventions. Thus, the following shell script can substitute
for Yacc:@refill
@end example
@end table
-@node Option Cross Key, VMS Invocation, Bison Options, Invocation
+@node Environment Variables, Option Cross Key, Bison Options, Invocation
+@section Environment Variables
+@cindex environment variables
+@cindex BISON_HAIRY
+@cindex BISON_SIMPLE
+
+Here is a list of environment variables which affect the way Bison
+runs.
+
+@table @samp
+@item BISON_SIMPLE
+@itemx BISON_HAIRY
+Much of the parser generated by Bison is copied verbatim from a file
+called @file{bison.simple}. If Bison cannot find that file, or if you
+would like to direct Bison to use a different copy, setting the
+environment variable @code{BISON_SIMPLE} to the path of the file will
+cause Bison to use that copy instead.
+
+When the @samp{%semantic_parser} delcaration is used, Bison copies from
+a file called @file{bison.hairy} instead. The location of this file can
+also be specified or overridden in a similar fashion, with the
+@code{BISON_HAIRY} environment variable.
+
+@end table
+
+@node Option Cross Key, VMS Invocation, Environment Variables, Invocation
@section Option Cross Key
Here is a list of options, alphabetized by long option, to help you find
\line{ --help \leaderfill -h}
\line{ --name-prefix \leaderfill -p}
\line{ --no-lines \leaderfill -l}
+\line{ --no-parser \leaderfill -n}
\line{ --output-file \leaderfill -o}
+\line{ --raw \leaderfill -r}
+\line{ --token-table \leaderfill -k}
\line{ --verbose \leaderfill -v}
\line{ --version \leaderfill -V}
\line{ --yacc \leaderfill -y}
--file-prefix=@var{prefix} -b @var{file-prefix}
--fixed-output-files --yacc -y
--help -h
---name-prefix -p
+--name-prefix=@var{prefix} -p @var{name-prefix}
--no-lines -l
+--no-parser -n
--output-file=@var{outfile} -o @var{outfile}
+--raw -r
+--token-table -k
--verbose -v
--version -V
@end example
Macro for the data type of @code{yylloc}; a structure with four
members. @xref{Token Positions, ,Textual Positions of Tokens}.
+@item yyltype
+Default value for YYLTYPE.
+
@item YYMAXDEPTH
Macro for specifying the maximum size of the parser stack.
@xref{Stack Overflow}.
Bison declaration to assign left associativity to token(s).
@xref{Precedence Decl, ,Operator Precedence}.
+@item %no_lines
+Bison declaration to avoid generating @code{#line} directives in the
+parser file. @xref{Decl Summary}.
+
@item %nonassoc
Bison declaration to assign nonassociativity to token(s).
@xref{Precedence Decl, ,Operator Precedence}.
Bison declaration to request a pure (reentrant) parser.
@xref{Pure Decl, ,A Pure (Reentrant) Parser}.
+@item %raw
+Bison declaration to use Bison internal token code numbers in token
+tables instead of the usual Yacc-compatible token code numbers.
+@xref{Decl Summary}.
+
@item %right
Bison declaration to assign right associativity to token(s).
@xref{Precedence Decl, ,Operator Precedence}.
Bison declaration to declare token(s) without specifying precedence.
@xref{Token Decl, ,Token Type Names}.
+@item %token_table
+Bison declaration to include a token name table in the parser file.
+@xref{Decl Summary}.
+
@item %type
Bison declaration to declare nonterminals. @xref{Type Decl, ,Nonterminal Symbols}.
A flag, set by actions in the grammar rules, which alters the way
tokens are parsed. @xref{Lexical Tie-ins}.
+@item Literal string token
+A token which consists of two or more fixed characters.
+@xref{Symbols}.
+
@item Look-ahead token
A token already read but not yet shifted. @xref{Look-Ahead, ,Look-Ahead Tokens}.
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