docs: private int dropUnusedTracks() {
  
  		List<PointTrack> all = trackerLeft.getAllTracks(null);
  		int num = 0;
  
  		for( PointTrack t : all ) {
  			LeftTrackInfo info = t.getCookie();
  			if( tick - info.lastInlier > thresholdRetire ) {
  				if( !trackerLeft.dropTrack(t) )
  					throw new IllegalArgumentException("failed to drop unused left track");
  				if( !trackerRight.dropTrack(info.right) )
  					throw new IllegalArgumentException("failed to drop unused right track");
  				num++;
  			}
  		}
  //		System.out.println("  total unused dropped "+num);
  
  		return num;
  	}