code: func (s *Session) closeStream(id uint32) {
  	s.streamLock.Lock()
  	if _, ok := s.inflight[id]; ok {
  		select {
  		case <-s.synCh:
  		default:
  			s.logger.Printf("[ERR] yamux: SYN tracking out of sync")
  		}
  	}
  	delete(s.streams, id)
  	s.streamLock.Unlock()
  }
docs: // closeStream is used to close a stream once both sides have
  // issued a close. If there was an in-flight SYN and the stream
  // was not yet established, then this will give the credit back.