static <A> L<Pt> matricesDifferingPoints(Matrix<A> a, Matrix<A> b) {
  if (a == null || b == null) null;
  assertSameSize(a, b);
  int w = a.getWidth(), h = a.getHeight();
  new L<Pt> l;
  for y to h:
    for x to w:
      if (!eq(a.get(x, y), b.get(x, y)))
        l.add(pt(x, y));
  ret l;
}