code: func (s *StanServer) processAckMsg(m *nats.Msg) {
  	ack := &pb.Ack{}
  	if ack.Unmarshal(m.Data) != nil {
  		if s.processCtrlMsg(m) {
  			return
  		}
  	}
  	c := s.channels.get(ack.Subject)
  	if c == nil {
  		s.log.Errorf("Unable to process ack seq=%d, channel %s not found", ack.Sequence, ack.Subject)
  		return
  	}
  	sub := c.ss.LookupByAckInbox(m.Subject)
  	if sub == nil {
  		return
  	}
  	s.processAck(c, sub, ack.Sequence, true)
  }
docs: // processAckMsg processes inbound acks from clients for delivered messages.