Depending on what you were doing, the undo playlist action in the main playlist editor window (better thought of as a temporary queue / scratch list vs what the media library playlists are like) can also be a simpler way to get things back to what had been there prior to what has been set be it loading an audio cd or another playlist, etc if that's what is wanted. Otherwise just add more items into the main playlist however you prefer be it internally or externally.
-dro