Get symbol from which the rule is rewrote.
        :param cls: Rule for which return the symbol.
        :return: Symbol from which the rule is rewrote.
        :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_toSymbol(cls):
        # type: (_MetaRule) -> object
        """
        Get symbol from which the rule is rewrote.
        :param cls: Rule for which return the symbol.
        :return: Symbol from which the rule is rewrote.
        :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:
            raise RuleNotDefinedException(cls)
        if len(cls.rules) > 1:
            raise CantCreateSingleRuleException(cls)
        right = cls.rules[0][1]
        if len(right) > 1:
            raise NotASingleSymbolException(right)
        return right[0]