code: func (s *SACKScoreboard) Copy() (sackBlocks []header.SACKBlock, maxSACKED seqnum.Value) {
  	s.ranges.Ascend(func(i btree.Item) bool {
  		sackBlocks = append(sackBlocks, i.(header.SACKBlock))
  		return true
  	})
  	return sackBlocks, s.maxSACKED
  }
docs: // Copy provides a copy of the SACK scoreboard.