docs: 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]