%{
#include <ctype.h>
  %}

%token DIGIT

%%
line : expr '\n' {printf("%d\n", $1);} /* $1 is first arg on RHS */
     ;

expr : expr '+' term {$$ = $1 + $3;}  /* $$ is attribute value of LHS */
     | term
     ;

term : term '*' factor  {$$ = $1 * $3;}  /* $$ is attribute value of LHS */
     | factor
     ;

factor : '(' factor ')' {$$ = $2;}
       | DIGIT
       ;

%%
       yylex() {
         int c;
         c = getchar();
         if (isdigit(c)) {
           yylval = c - '0';
           return DIGIT;
         }
         return c;
       }