static LPair<IntRange, Bool> bitSetStreaksAndNonStreaks(BitSet bs, int n) {
  int start = 0;
  Bool last = null;
  new LPair<IntRange, Bool> out;
  for i to n: {
    Bool result = bs.get(i);
    if (last != null && neq(result, last)) {
      out.add(pair(intRange(start, i), last));
      start = i;
    }
    last = result;
  }
  if (start < n)
    out.add(pair(intRange(start, n), last));
  ret out; 
}