// type parameter is rect + FEN
sclass ChessOCR_TwoStageRecognizer extends SteppableAndBest<Pair<Rect, S>> {
  replace Segmenter with IF1<BufferedImage, L<Rect>>.
  
  // input
  BufferedImage inputImage;
  Iterator<? extends Segmenter> segmenterStream;
  L<Rect> rectQueue = new LinkedList;
  ChessPieceRecognizer pieceRecognizer;
  
  // internal
  new Set<Rect> rectsTried;

  *() {}
  *(ChessPieceRecognizer *pieceRecognizer, BufferedImage *inputImage, Iterator<? extends Segmenter> *segmenterStream) {}
  
  public bool step() {
    if (nempty(rectQueue))
      ret true with tryRect(popFirst(rectQueue));
    if (segmenterStream.hasNext())
      ret true with addAll(rectQueue, segmenterStream.next().get(inputImage));
    false;
  }
  
  void tryRect(Rect r) {
    r = rectSetHeightToWidth(r); // square it
    if (!rectsTried.add(r)) ret; // seen
    BufferedImage board = clipBufferedImage(inputImage, r);
    S fen, double score = unpair chessOCR_recognizeBoard_FENAndScore(board, pieceRecognizer);
    best.put(pair(r, fen), score);
  }
  
  void verbose(bool verbose) {
    best.verboseNewBest = verbose;
  }
}