static LS toLines_nOnly_reversible(S s) {
  new LS lines;
  if (s == null) ret lines;
  int start = 0, n = s.length();
  while (start < n) {
    int i = smartIndexOf(s, '\n', start);
    lines.add(s.substring(start, i));
    start = i+1;
  }
  ret lines;
}