// 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);

}