// images must have same size
static BWImage averageOfBinaryImages(L<? extends IBinaryImage> l) {
  int n = l(l);
  if (n == 0) null;
  var it = iterator(l);
  var img = it.next();
  int w = img.getWidth(), h = img.getHeight();
  int[] pixels = new[w*h];
  while true {
    int i = 0;
    for y to h:
      for x to w: {
        if (img.getBoolPixel(x, y))
          pixels[i]++;
        i++;
      }
    if (!it.hasNext()) break;
    img = it.next();
  }
  byte[] bytes = new[w*h];
  for i over pixels:
    bytes[i] = (byte) iround(pixels[i]*(255/n));
  ret new BWImage(w, h, bytes);
}