docs: def _penalize_client(self, client):
          """Place client in the penalty box.
  
          :param client: Client object
          """
          if client in self.active_clients:  # hasn't been removed yet
              log.warning("%r marked down.", client)
              self.active_clients.remove(client)
              self.penalty_box.add(client)
          else:
              log.info("%r not in active client list.")