static LS allPrefixesInNavigableSet(S s, NavigableSet<S> set) {
  if (set == null || s == null) null;
  new LS prefixes;
  while licensed {
    S key = set.floor(s);
    if (key == null) break; // s is in front of whole set => no prefix in there
    int n = lCommonPrefix(key, s);
    if (n == l(key)) {
      prefixes.add(key); // found a prefix!
      --n;
    }
    s = takeFirst(s, n); // shorten and try again
  }
  ret prefixes;
}