!7

p-subst {
  repeat with sleep 1 {
    currentImage(newBufferedImage(200, 200, Color.gray));
    Rect r = randomRect();
    r.h = r.h & ~1; // make even
    Pair<Rect> p = tossCoin()
      ? splitRectInVerticalHalves(r)
      : splitRectInHorizontalHalves(r);
    if (oneInTwoChance()) p = reversePair(p);
    fillRect(p.a, Color.black);
    fillRect(p.b, Color.white);
    quickShowImage();
  }
}