};

  private final NavigableSet<E> delegate;

  public SafeTreeSet() {
    this(new TreeSet<E>());
  }

  public SafeTreeSet(Collection<? extends E> collection) {
    this(new TreeSet<E>(collection));
  }

  public SafeTreeSet(Comparator<? super E> comparator) {
    this(new TreeSet<E>(comparator));
  }

  public SafeTreeSet(SortedSet<E> set) {
    this(new TreeSet<E>(set));
  }

            .named("SafeTreeSet with natural comparator")
            .createTestSuite());
    suite.addTest(
        SetTestSuiteBuilder.using(
                new TestStringSetGenerator() {
                  @Override
                  protected Set<String> create(String[] elements) {
                    NavigableSet<String> set = new SafeTreeSet<>(Ordering.natural().nullsFirst());

        NavigableSetTestSuiteBuilder.using(
                new TestStringSetGenerator() {
                  @Override
                  protected Set<String> create(String[] elements) {
                    SafeTreeSet<String> set = new SafeTreeSet<>(asList(elements));
                    return unmodifiableNavigableSet(set);
                  }

                  @Override
                  public List<String> order(List<String> insertionOrder) {