sS fastJoin(LS l) {
  if (empty(l)) ret "";
  int n = 0;
  for (S s : l) n += s.length();
  char[] buf = new[n];
  n = 0;
  for (S s : l) {
    int x = s.length();
    s.getChars(0, x, buf, n);
    n += x;
  }
  ret new S(buf);
}