code: func (l *PacketList) InsertAfter(v Packet, mark *PacketElement) *PacketElement {
  	if mark.list != l {
  		return nil
  	}
  	// see comment in List.Remove about initialization of l
  	return l.insertValue(v, mark)
  }
docs: // InsertAfter inserts a new element e with value v immediately after mark and returns e.
  // If mark is not an element of l, the list is not modified.
  // The mark must not be nil.