docs: 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]