%{
/* acfg.y */
#include <stdio.h>
#include <ctype.h>
%}

%start S

%token 'a'
%token 'b'

%%   /* beginning of rules section (printf statements optional) */

S	:	'a' A S
			{	printf("S -> aAS \n"); }
	|	'a'
			{	printf("S -> a \n"); }
	;

A	:	S 'b' A
			{	printf("A -> SbA \n"); }
	|	S S
			{	printf("A -> SS a \n"); }
	|	'b' 'a'
			{	printf("A -> ba \n"); }
	;

%%  /* start of program, need to provide yylex that delivers tokens */
yylex()
{
    int c;

    c=getchar();
    printf("read %c \n",c);  /* can be commented out */
    if(!islower(c)) c=0;     /* cause parser to stop */
    yylval=c;
    return c;
}


/* FYI in CMSC 451    equivalent grammar   P83

  G = (V, T, P, S)   V = { A, S }  T= { a, b }  S = S
  P is                 or
       S -> aAS           S -> aAS | a
       S -> a
       A -> SbA           A -> SbA | SS | ba
       A -> SS
       A -> ba
*/