sclass ChessOCR_TwoStageRecognizer implements Steppable {
  // input
  BufferedImage inputImage;
  Iterator<Rect> rectStream;
  ChessPieceRecognizer pieceRecognizer;
  
  // internal
  new Set<Rect> rectsTried;

  // output
  new Best<Pair<Rect, S>> best; // rect + FEN

  *() {}
  *(ChessPieceRecognizer *pieceRecognizer, BufferedImage *inputImage, Iterator<Rect> *rectStream) {}
  
  public bool step() {
    if (!rectStream.hasNext()) false;
    ret true with tryRect(rectStream.next());
  }
  
  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);
  }
}