code: func (s *SACKScoreboard) IsSACKED(r header.SACKBlock) bool {
  	found := false
  	s.ranges.DescendLessOrEqual(r, func(i btree.Item) bool {
  		sacked := i.(header.SACKBlock)
  		if sacked.End.LessThan(r.Start) {
  			return false
  		}
  		if sacked.Contains(r) {
  			found = true
  			return false
  		}
  		return true
  	})
  	return found
  }
docs: // IsSACKED returns true if the a given range of sequence numbers denoted by r
  // are already covered by SACK information in the scoreboard.