static Pair<Rect, S> chessOCR_demoTwoStageRecognizer(BufferedImage img) {
  replace PS with ParameterizedSegmenter.
  
  ImageSurface is = showImage(img);

  PS seg1 = parameterizedSegmenterFromAGIBlue("find chess board #4 #1");
  Iterator<PS> seg2 = chessOCR_varySegmenterGain(seg1);
  Iterator<PS> seg3 = nestedIterator(seg2, func(PS seg) -> Iterator<PS> {
    chessOCR_adaptSegmenterToMainImageColors(img, seg, maxColors := 4)
  });
  
  ChessOCR_TwoStageRecognizer rec = new(chessOCR_pieceRecognizer(), img, seg3);
  rec.verbose(true);

  new Var<Rect> last;
  while ping (rec.step())
    if (setVar_trueIfChanged(last, rec.bestBoardLocation()))
      overlaySelectionsOnImageSurface(is, ll(last!));
  print(rec.best);
  ret rec!;
}