// images must have same size

static int[] sumOfBinaryImages(L<? extends IBinaryImage> images) {

  int n = l(images);

  if (n == 0) null;

  var it = iterator(images);

  var img = it.next();

  int w = img.getWidth(), h = img.getHeight();

  int[] pixels = new[w*h];

  int nBytes = (w*h+7)/8;

  var pingSource = pingSource();

  while (true) {

    ping(pingSource);

    Image2B realizedImage = img.toImage2B();

    byte[] imgPixels = realizedImage.pixels;

    int j = 0;

    for i to nBytes: {

      int b = imgPixels[i];

      for bit to 8: {

        pixels[j++] += b & 1;

        b >>= 1;

      }

    }

    if (!it.hasNext()) break;

    img = it.next();

  }

  ret pixels;

}