code: def p_expr_add_term(self, args):
          ' expr ::= expr ADD_OP term '
          op = 'add' if args[1].attr == '+' else 'subtract'
          return AST(op, [args[0], args[2]]f p_expr_add_term(self, args):
          ' expr ::= expr ADD_OP term '
          op = 'add' if args[1].attr == '+' else 'subtract'
          return AST(op, [args[0], args[2]])
docs: expr ::= expr ADD_OP term