* implemented in terms of sortedDelegate (except comparator()).
   */
  ImmutableSortedSet(Comparator<? super E> comparator) {
    this(Sets.newTreeSet(comparator));
  }

  ImmutableSortedSet(SortedSet<E> sortedDelegate) {
    super(sortedDelegate);
    this.sortedDelegate = Collections.unmodifiableSortedSet(sortedDelegate);
  }

  public Comparator<? super E> comparator() {

    // TreeMap.navigatbleKeySet().
    return ImmutableSortedSet.copyOf(comparator, sortedDelegate.keySet());
  }

  public Comparator<? super K> comparator() {
    return comparator;
  }

  @CheckForNull
  public K firstKey() {
    return sortedDelegate.firstKey();
  }

  @CheckForNull
  public K lastKey() {
    return sortedDelegate.lastKey();
  }

  @CheckForNull
  K higher(K k) {