code: @Override
  	public boolean dropTrack(PointTrack track) {
  		if( !tracksAll.remove(track) )
  			return false;
  
  		if( !sets[track.setId].tracks.remove(track) ) {
  			return false;
  		}
  		// the track may or may not be in the active list
  		tracksActive.remove(track);
  		tracksInactive.remove(track);
  		// it must be in the all list
  		// recycle the data
  		unused.add(track);
  		return true;
  	}
docs: Remove from active list and mark so that it is dropped in the next cycle
  
  @param track The track which is to be dropped