code: func (m *ReceivedMessageImpl) Ack(err error) {
  	ackMsg := &proto.GossipMessage{
  		Nonce: m.GetGossipMessage().Nonce,
  		Content: &proto.GossipMessage_Ack{
  			Ack: &proto.Acknowledgement{},
  		},
  	}
  	if err != nil {
  		ackMsg.GetAck().Error = err.Error()
  	}
  	m.Respond(ackMsg)
  }
docs: // Ack returns to the sender an acknowledgement for the message