static L<Pt> neighborhood8(int x, int y) {
  ret ll(new Pt(x-1, y-1), new Pt(x, y-1), new Pt(x+1, y-1),
    new Pt(x-1, y), new Pt(x+1, y),
    new Pt(x-1, y+1), new Pt(x, y+1), new Pt(x+1, y+1));
}