%debug
%{
# include <stdio.h>
# include <stdlib.h>
  static void yyerror (const char *msg);
  static int yylex (void);
# define YYDEBUG         1
# define YYERROR_VERBOSE 1
%}
%%
input:
  a_b 'b'
| a_c 'c'
| error '!' { fprintf (stderr, "OK\n"); }
;
a_b: 'a' { YYERROR; };
a_c: 'a' { YYERROR; };
%%
static int
yylex (void)
{
  static const char *input = "a!";
  return *input++;
}

static void
yyerror (const char *msg)
{
  fprintf (stderr, "%s\n", msg);
}

int
main (void)
{
  yydebug = 1;
  return yyparse ();
}