static int[] bitSetToIntArray(BitSet bs) {
  if (bs == null) null;
  int n = bs.cardinality();
  int[] result = new[n];
  int j = 0;
  for (int i = bs.nextSetBit(0); i >= 0; i = bs.nextSetBit(i+1))
    result[j++] = i;
  ret result;
}