static <A> L<A> selectRandom_fast(L<A> l, int k) {
  int n = l(l);
  l = cloneList(l);
  for (int i = n - 1; i >= n-k; i--) {
    int j = random(i+1);
    swapElements(l, i, j);
  }
  ret subList(l, n-k);
}