%{
void yyerror (char *s);
#include <stdio.h>
#include <stdlib.h>
yydebug=1;
%}
%token BEGINT ENDT PROGRAM OUTPUT INPUT IDENTIFIER INT

%%
        program :   PROGRAM declaration_s BEGINT statement_s ENDT ';'               {printf("This is a valid program.\n");
                                                                                                exit(0);
                                                                                }
                ;
declaration_s   :      type ':' IDENTIFIER declaration
                ;
type            :      INT
                ;
declaration      :     ',' IDENTIFIER declaration
                 |     ';'
                 ;
statement_s      :     assign_statement
                 |     output_statement
                 |     input_statement
                 ;
assign_statement :     IDENTIFIER '=' expression ';'
                 ;
output_statement :     OUTPUT IDENTIFIER declaration
                 ;
input_statement  :     INPUT IDENTIFIER declaration
                 ;
expression       :     expression '+' IDENTIFIER
                 |     IDENTIFIER
                 ;
%%