svoid saveScreenClip(BWImage img) {
  S md5 = ocrMD5OfBWImage(img);
  File f = screenClipFileForMD5(md5);
  if (!f.exists()) {
    BufferedImage bufferedImage = img.getBufferedImage();
    // verify 1
    assertEquals("BufferedImage md5", md5, ocrMD5OfBufferedImage(bufferedImage));
    savePNG(f, bufferedImage);
    bufferedImage = loadBufferedImage(f);
    // verify 2
    assertEquals("loaded", md5, ocrMD5OfBufferedImage(bufferedImage));
  }
}