docs: def _get_rules(cls):
          # type: (_MetaRule) -> List[(List[object], List[object])]
          """
          Get rules on the Rule class.
          :param cls: Rule for which return the rules.
          :return: Rules 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.
          """
          cls._traverse = True
          r = cls.rule
          cls._traverse = False
          return [r]