docs: private int dropUnusedTracks() {
  
  		List<PointTrack> all = tracker.getAllTracks(null);
  		int num = 0;
  
  		for (PointTrack t : all) {
  			VoTrack p = t.getCookie();
  			if (tick - p.lastInlier > thresholdRetire) {
  				tracker.dropTrack(t);
  				num++;
  			}
  		}
  
  		return num;
  	}