code: def add(self, event, message=None):
          
          def decorator(func):
              if isinstance(message, (list, tuple)):
                  for it in message:
                      self.__add_handler(func, event, message=it)
              else:
                  self.__add_handler(func, event, message=message)
  
              return func
  
          return decorator
docs: [Decorator] Add handler method.
  
          :param event: Specify a kind of Event which you want to handle
          :type event: T <= :py:class:`linebot.models.events.Event` class
          :param message: (optional) If event is MessageEvent,
              specify kind of Messages which you want to handle
          :type: message: T <= :py:class:`linebot.models.messages.Message` class
          :rtype: func
          :return: decorator