code: func TrimSACKBlockList(sack *SACKInfo, rcvNxt seqnum.Value) {
  	n := 0
  	for i := 0; i < sack.NumBlocks; i++ {
  		if sack.Blocks[i].End.LessThanEq(rcvNxt) {
  			continue
  		}
  		if sack.Blocks[i].Start.LessThan(rcvNxt) {
  			// Shrink this SACK block.
  			sack.Blocks[i].Start = rcvNxt
  		}
  		sack.Blocks[n] = sack.Blocks[i]
  		n++
  	}
  	sack.NumBlocks = n
  }
docs: // TrimSACKBlockList updates the sack block list by removing/modifying any block
  // where start is < rcvNxt.