code: func (r *receiver) acceptable(segSeq seqnum.Value, segLen seqnum.Size) bool {
  	rcvWnd := r.rcvNxt.Size(r.rcvAcc)
  	if rcvWnd == 0 {
  		return segLen == 0 && segSeq == r.rcvNxt
  	}
  
  	return segSeq.InWindow(r.rcvNxt, rcvWnd) ||
  		seqnum.Overlap(r.rcvNxt, rcvWnd, segSeq, segLen)
  }
docs: // acceptable checks if the segment sequence number range is acceptable
  // according to the table on page 26 of RFC 793.