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.")