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.

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]