docs: def nack(self, frame):
          """
          Handles the NACK command: Unacknowledges receipt of a message.
          For now, this is just a placeholder to implement this version of the protocol
          """
          if not frame.headers.get('message-id'):
              raise ProtocolError("No message-id specified for NACK command.")
          if not frame.headers.get('subscription'):
              raise ProtocolError("No subscription specified for NACK command.")