code: def _get_fromSymbol(cls):
          # type: (_MetaRule) -> object
          
          if cls._traverse:
              raise RuleNotDefinedException(cls)
          if len(cls.rules) > 1:
              raise CantCreateSingleRuleException(cls)
          left = cls.rules[0][0]
          if len(left) > 1:
              raise NotASingleSymbolException(left)
          return left[0]
docs: Get symbol to which the rule is rewrote.
          :param cls: Rule for which return the symbol.
          :return: Symbol to 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 right is more.