use canvas

w = 800 h = 600
window("Fractal rectangle demo", w, h)
fractal(w/2, h/2, w/2)
repaint()

def rect(x, y, w, h) {
  line(x, y, x + w, y)
  line(x + w, y, x + w, y + h)
  line(x, y + h, x + w, y + h)
  line(x, y, x, y + h)
}

def fractal(cx, cy, size) {
  if size >= 2 {
    s2 = size / 2
    color(0, 0, 255 - size * 255 / w/2)
    rect(cx-s2, cy-s2, size, size)
    fractal(cx-s2, cy-s2, s2)
    fractal(cx+s2, cy-s2, s2)
    fractal(cx-s2, cy+s2, s2)
    fractal(cx+s2, cy+s2, s2)
  }
}