## Comments

    // Single-line comment
    /* Multi-line
    comment */

## C prologue and Bison declarations

    %{
      #include <stdio.h>
      #include <math.h>
      int yylex (void);
      void yyerror (char const *);
    %}

    %define api.value.type {double}
    %token NUM
    %union { char *string; }
    %%
    %%

## Grammar rules

    %%
    exp:
      NUM           { $$ = $1;           }
    | exp exp '+'   { $$ = $1 + $2;      }
    | exp exp '-'   { $$ = $1 - $2;      }
    | exp exp '*'   { $$ = $1 * $2;      }
    | exp exp '/'   { $$ = $1 / $2;      }
    | exp exp '^'   { $$ = pow($1, $2);  }  /* Exponentiation */
    | exp 'n'       { $$ = -$1;          }  /* Unary minus    */
    ;

    $@1: %empty { a(); };
    $@2: %empty { c(); };
    $@3: %empty { d(); };
    exp: $@1 "b" $@2 $@3 "e" { f(); };
    %%

## Full example

    /* Mini Calculator */
    /* calc.y */

    %{
    #include "heading.h"
    int yyerror(char *s);
    int yylex(void);
    %}

    %union{
      int		int_val;
      string*	op_val;
    }

    %start	input

    %token	<int_val>	INTEGER_LITERAL
    %type	<int_val>	exp
    %left	PLUS
    %left	MULT

    %%

    input:		/* empty */
    		| exp	{ cout << "Result: " << $1 << endl; }
    		;

    exp:		INTEGER_LITERAL	{ $$ = $1; }
    		| exp PLUS exp	{ $$ = $1 + $3; }
    		| exp MULT exp	{ $$ = $1 * $3; }
    		;

    %%

    int yyerror(string s)
    {
      extern int yylineno;	// defined and maintained in lex.c
      extern char *yytext;	// defined and maintained in lex.c

      cerr << "ERROR: " << s << " at symbol \"" << yytext;
      cerr << "\" on line " << yylineno << endl;
      exit(1);
    }

    int yyerror(char *s)
    {
      return yyerror(string(s));
    }