static S fixNewLines(S s) {
  int i = indexOf(s, '\r');
  if (i < 0) ret s;
  int l = s.length();
  StringBuilder out = new(l);
  for i to l: {
    char c = s.charAt(i);
    if (c != '\r')
      out.append(c);
    else {
      out.append('\n');
      if (i+1 < l && s.charAt(i+1) == '\n') ++i;
    }
  }
}