function maximum(Set s) {
  if (s.length == 0) {
    return null;
  } else if (s.length == 1) {
    return s[0];
  } else if (s[0] > s[s.length - 1]) {
    s = s.removeLast();
    return maximum(s);
  } else {
    s = s.removeFirst();
    return maximum(s);
  }
}