static BWImage mergeBWImagesVertically(BWImage... images) {
  ret mergeBWImagesVertically(toList(images));
}

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