boolean isDigit(char ch) {
   return ch >= '0' && ch <= '9';
}

void p() {
   e();
   while ( isDigit(token) ) 
      e();
}

void e() {
   t();
   while ( token == '+' || token == '-' ) {
      char op = token;
      nextToken();
      t();
      sop("pop R1");
      sop("pop R0");
      if ( op == '+' )
          sop("add R0, R1");
      else
          sop("sub R0, R1");
      sop("push R0");   }
}

void t() {
   f();
   while ( token == '*' ) {
      char op = token;
      nextToken();
      f();
      sop("pop R1");
      sop("pop R0");
      sop("mul R0, R1");
      sop("push R0");
   }
}

void f() {
   if ( isDigit(token) ) {
      sop("mov R0, " + token);
      sop("push R0");
      nextToken();
   }
   else
      error();
}