docs: def is_registered(self, event_type, callback, details_filter=None):
          """Check if a callback is registered.
  
          :param event_type: event type callback was registered to
          :param callback: callback that was used during registration
          :param details_filter: details filter that was used during
                                 registration
  
          :returns: if the callback is registered
          :rtype: boolean
          """
          listeners = self._topics.get(event_type, [])
          for listener in listeners:
              if listener.is_equivalent(callback, details_filter=details_filter):
                  return True
          return False