static <A, B extends Iterable<A>> ItIt<L<A>> outerProduct_faster(L<B> lists) {
  ret mapI fromSimplyLinkedList(outerProduct_faster(toSimplyLinkedList(lists)));
}

static <A, B extends Iterable<A>> ItIt<SimplyLinkedList<A>> outerProduct_faster(SimplyLinkedList<B> lists) {
  if (lists == null) ret singletonIterator(null);
  SimplyLinkedList<B> lists2 = lists.tail;
  ret nestedIterator(lists.head, func(A a) -> Iterator<SimplyLinkedList<A>> {
    mapI(outerProduct_faster(lists2), l -> SimplyLinkedList.cons(a, l))
  });
}

Began life as a copy of #1023780