static RGBImage mergeImagesVertically(RGBImage... images) {
  ret mergeImagesVertically(toList(images));
}

static RGBImage mergeImagesVertically(L<RGBImage> images, O... _) {
  optPar int spacing = imageMergeSpacing();
  RGB spaceColor = optPar spaceColor(_, RGB(0.9f));
  int h = intSum(images, "height")+(l(images)-1)*spacing, w = intMax(images, "width");
  RGBImage out = new RGBImage(w, h, spaceColor);
  int y = 0;
  for i over images: {
    RGBImage img = images.get(i);
    int x = (w-img.w())/2;
    copyRGBImage(img, 0, 0, out, x, y);
    y += img.h() + spacing;
  }
  ret out;
}