static L<IntRange> bitSetStreaks(BitSet bs) {
  new L<IntRange> streaks;
  if (bs == null) ret streaks;
  int n = bs.length();
  int i = -1;
  while true {
    i = bs.nextSetBit(i+1);
    if (i < 0) break;
    int j = bs.nextClearBit(i+1);
    streaks.add(intRange(i, j));
    i = j;
  }
  ret streaks;
}