code: def p_identifier_group_op(self, p):
          
          p[1].extend([p[2]])
          if len(p) > 3:
              p[1].extend(p[3])
          p[0] = p[1]
docs: identifier_group          : identifier_group child_selector ident_parts
                                        | identifier_group '+' ident_parts
                                        | identifier_group general_sibling_selector ident_parts
                                        | identifier_group '*'