%{
/* acfg2.y */
#include <stdio.h>
#include <ctype.h>
%}

%start S

%token 'a'
%token 'b'

%%   /* beginning of rules section */

S	:	'a' B
			{	printf("S -> aB \n"); }
	|	'b' A
			{	printf("S -> Ba \n"); }
	;

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

B	:	'b'
			{	printf("B -> b \n"); }
	|	'b' S
			{	printf("B -> bS \n"); }
	|	'a' B B
			{	printf("B -> aBB \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;
}

/* G = ( V, T, P, S )  V = { A, B, S }  T = { a, b }  S = S
   P is   S -> aB | bA
          A -> a  | aS | bAA
          B -> b  | bS | aBB
   any string with equal number of a's and b's */