static <A, B extends Iterable<A>> ItIt<L<A>> outerProduct(L<B> lists) {
  if (empty(lists)) ret singletonIterator(new L);
  L<B> lists2 = dropFirst(lists);
  ret nestedIterator(first(lists), func(A a) -> Iterator<L<A>> {
    mapI(outerProduct(lists2), l -> itemPlusList(a, l))
  });
}

Began life as a copy of #1023780