// returns selected, rest
static <A> Pair<L<A>> selectRandom2(L<A> list, int n) {
  if (l(list) < n) null;
  new L<A> l;
  list = cloneList(list);
  repeat n {
    int i = rand(l(list));
    l.add(list.get(i));
    list.remove(i);
  }
  ret pair(l, list);
}