code: def receive_empty(self, empty, transaction):
          
          if empty.type == defines.Types["RST"]:
              host, port = transaction.request.source
              key_token = hash(str(host) + str(port) + str(transaction.request.token))
              logger.info("Remove Subscriber")
              try:
                  del self._relations[key_token]
              except KeyError:
                  pass
              transaction.completed = True
          return transaction
docs: Manage the observe feature to remove a client in case of a RST message receveide in reply to a notification.
  
          :type empty: Message
          :param empty: the received message
          :type transaction: Transaction
          :param transaction: the transaction that owns the notification message
          :rtype : Transaction
          :return: the modified transaction