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

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