public Track add(Track track) {
   synchronized (this) {
     Track copy;
     if ((copy = getTrack(track)) == null) {
       copy = new Track((Element) doc.importNode(track.getElement(), false));
       docElt.appendChild(copy.getElement());
       tracks.add(copy);
       hash.put(copy.getKey(), copy);
     }
     return copy;
   }
 }
 public void remove(Track track) {
   synchronized (this) {
     docElt.removeChild(track.getElement());
     tracks.remove(track);
     hash.remove(track.getKey());
   }
 }