svoid test_allSubRectsOfSizeIterator() {
  Rect full = rect(1, 2, 5, 4);
  assertEqualsVerbose(ll(full), asList(allSubRectsOfSizeIterator(5, 4, full)));
  assertEqualsVerbose(ll(
    rect(1, 2, 5, 3), rect(1, 3, 5, 3)
  ), asList(allSubRectsOfSizeIterator(5, 3, full)));
}