static RGBImage renderColorPredicateOnTestImages(F1<Int, Bool> pred, L<RGBImage> testImages) {
 new L<RGBImage> out;
  for (RGBImage img : testImages) {
    out.add(img);
    out.add(renderColorPredicate(pred, img));
  }
  ret mergeImagesVertically(out);
}