code: def add(self, handler, allow_dupe=False, send_event=True):
          
          self._add(handler, allow_dupe=allow_dupe, send_event=send_event)
          return handler
docs: Add handler instance and attach any events to it.
  
          :param object handler: handler instance
          :param bool allow_dupe: If True, allow registering a handler more than once.
          :return object: The handler you added is given back so this can be used as a decorator.