Get left part of the rule.
        :param cls: Rule for which return the left side.
        :return: Symbols on the left side of the array.
        :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_left(cls):
        # type: (_MetaRule) -> List[object]
        """
        Get left part of the rule.
        :param cls: Rule for which return the left side.
        :return: Symbols on the left side of the array.
        :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.
        """
        if cls._traverse:
            return [cls.fromSymbol]
        if len(cls.rules) > 1:
            raise CantCreateSingleRuleException(cls)
        return cls.rules[0][0]