static <A> L<A> selectRandom(L<A> list, int n) {
  if (l(list) < n) null;
  new L<A> l;
  list = cloneList(list);
  n = min(n, l(list));
  repeat n {
    int i = rand(l(list));
    l.add(list.get(i));
    list.remove(i);
  }
  ret l;
}