static IterableIterator<S> lines_iterator(fS s) {
  if (s == null) ret emptyIterableIterator();
  ret iteratorFromFunction(new F0<S> {
    int start = 0;
    
    S get() {
      int i = toLines_nextLineBreak(s, start);
      if (i < 0) {
        S toReturn = s.length() > start ? s.substring(start) : null;
        start = s.length();
        ret toReturn;
      }

      S toReturn = s.substring(start, i);
      if (s.charAt(i) == '\r' && i+1 < s.length() && s.charAt(i+1) == '\n')
        i += 2;
      else
        ++i;
      start = i;
      ret toReturn;
    }
  });
}