code: func (s *stream) acceptable(segSeq seqnum.Value, segLen seqnum.Size) bool {
  	wnd := s.una.Size(s.end)
  	if wnd == 0 {
  		return segLen == 0 && segSeq == s.una
  	}
  
  	// Make sure [segSeq, seqSeq+segLen) is non-empty.
  	if segLen == 0 {
  		segLen = 1
  	}
  
  	return seqnum.Overlap(s.una, wnd, segSeq, segLen)
  }
docs: // acceptable determines if the segment with the given sequence number and data
  // length is acceptable, i.e., if it's within the [una, end) window or, in case
  // the window is zero, if it's a packet with no payload and sequence number
  // equal to una.