static int[] intersectIntArrayAndBitSet(int[] array, BitSet bs) {
  if (array == null || bs == null) null;
  int n = array.length;
  IntBuffer out = new(n);
  for i to n: {
    int x = array[i];
    if (bs.get(x))
      out.add(x);
  }
  ret out.toArray();
}