docs: def send_ack(self):
          """Send an ack message"""
  
          if self.last_ack == self.proto.max_id:
              return
          LOGGER.debug("ack (%d)", self.proto.max_id)
          self.last_ack = self.proto.max_id
          self.send_message(f"4{to_json([self.proto.max_id])}")