private final SortedSet<E> unmodifiableDelegate;

    UnmodifiableNavigableSet(NavigableSet<E> delegate) {
      this.delegate = checkNotNull(delegate);
      this.unmodifiableDelegate = Collections.unmodifiableSortedSet(delegate);
    }

    @Override
    protected SortedSet<E> delegate() {
      return unmodifiableDelegate;
    }

    @Override
    @CheckForNull