static Puzzle makeClickPuzzle(S instruction, L<RGBImage> images, L<Rect> solutions) {
  ret makeClickPuzzle(instruction, null, images, solutions);
}

static Puzzle makeClickPuzzle(S instruction, S nullAnswer,
  L<RGBImage> images, L<Rect> solutions) {
  // select image
  int i = random(l(images));
  RGBImage mainImage = images.get(i);
  Rect solution = solutions.get(i);
  
  Rect clip;
  if (solution != null) {
    // select random portion
    clip = selectRandomPartContaining(mainImage, solution, w, h);
    solution = translateRect(solution, -clip.x, -clip.y);
  } else {
    clip = selectRandomPart(mainImage, w, h);
    if (clip == null) fail("No fit: " + w + "/" + h + " vs " + mainImage.w() + "/" + mainImage.h());
  }
  RGBImage img = mainImage.clip(clip);
  
  // return puzzle
  ret new Puzzle(instruction, nullAnswer, img, llNonNulls(solution));
}