code: def _get_rule(cls):
          # type: (_MetaRule) -> (List[object], List[object])
          
          if cls._traverse:
              return (cls.left, cls.right)
          if len(cls.rules) > 1:
              raise CantCreateSingleRuleException(cls)
          return cls.rules[0]
docs: Get rule on the Rule class.
          :param cls: Rule for which return the rule.
          :return: Rule inside the Rule class.
          :raise RuleNotDefinedException: If the rule is not defined.
          :raise CantCreateSingleRuleException: If the rule consists of more rules.
          :raise NotASingleSymbolException: If number of symbols on the left is more.