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

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