static ItIt<Pt> pixelIterator(Rect r) {
  ret new ItIt<Pt> {
    int x2 = r.x2(), y2 = r.y2();
    int x = r.x, y = r.y;
    
    public bool hasNext() {
      ret y < y2 && x < x2;
    }
    
    public Pt next() {
      Pt p = pt(x, y);
      if (++x >= x2) { x = r.x; ++y; }
      ret p;
    }
  };
}