static ItIt<Int> lineStartIndexesIterator(S s) {
  ret new ItIt<Int> {
    int i;
    
    public bool hasNext() {
      ret i >= 0;
    }
    
    public Int next() {
      int j = i;
      i = indexOf(s, '\n', i);
      if (i >= 0) ++i;
      ret j;
    }
  };
}

Began life as a copy of #1005051