static L<Pt> ptGrid(int x1, int y1, int w, int h, int stepX, int stepY) {
  new L<Pt> l;
  for (int y = 0; y < h; y += stepY)
    for (int x = 0; x < w; x += stepX)
      l.add(pt(x1+x, y1+y));
  ret l;
}