code: func (s *Service) Subscribe(topic string, qos packet.QOS) SubscribeFuture {
  	return s.SubscribeMultiple([]packet.Subscription{
  		{Topic: topic, QOS: qos},
  	})
  }
docs: // Subscribe will send a Subscribe packet containing one topic to subscribe. It
  // will return a SubscribeFuture that gets completed once the acknowledgements
  // have been received.