docs: def trigger_all_callbacks(self, callbacks=None):
          """Trigger callbacks for all keys on all or a subset of subscribers.
  
          :param Iterable callbacks: list of callbacks or none for all subscribed
          :rtype: Iterable[tornado.concurrent.Future]
          """
          return [ret
                  for key in self
                  for ret in self.trigger_callbacks(key, callbacks=None)]