static <A> ItIt<L<A>> allSubsetsOfSize_iterator(int m, L<A> objects) {
  int n = l(objects);
  ret mapI(allSpreads(n-m, m+1), spread -> {
    L<A> out = emptyList(m);
    int i = -1;
    for j to m: {
      i += spread.get(j)+1;
      out.add(objects.get(i));
    }
    ret out;
  });
}