.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());

            .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());

                  }

                  @Override
                  public List<String> order(List<String> insertionOrder) {
                    return Ordering.natural().sortedCopy(insertionOrder);
                  }
                })
            .named("ImmutableSortedMultiset")
            .withFeatures(
                CollectionSize.ANY,

                new TestStringSetGenerator() {
                  @Override
                  protected Set<String> create(String[] elements) {
                    SafeTreeSet<String> set = new SafeTreeSet<>(Ordering.natural().nullsFirst());
                    Collections.addAll(set, elements);
                    return new StandardImplForwardingNavigableSet<>(set);
                  }

                  @Override

 *
 * @author Louis Wasserman
 */
@GwtIncompatible
public final class SafeTreeSet<E> implements Serializable, NavigableSet<E> {
  @SuppressWarnings("unchecked")
  private static final Comparator<Object> NATURAL_ORDER =
      new Comparator<Object>() {
        @Override
        public int compare(Object o1, Object o2) {
          return ((Comparable<Object>) o1).compareTo(o2);
        }
      };

 *
 * @author Louis Wasserman
 */
@GwtIncompatible
public final class SafeTreeMap<K, V> implements Serializable, NavigableMap<K, V> {
  @SuppressWarnings("unchecked")
  private static final Comparator<Object> NATURAL_ORDER =
      new Comparator<Object>() {
        @Override
        public int compare(Object o1, Object o2) {
          return ((Comparable<Object>) o1).compareTo(o2);
        }
      };

 *
 * @author Louis Wasserman
 */
@GwtIncompatible
public final class SafeTreeMap<K, V> implements Serializable, NavigableMap<K, V> {
  @SuppressWarnings("unchecked")
  private static final Comparator<Object> NATURAL_ORDER =
      new Comparator<Object>() {
        @Override
        public int compare(Object o1, Object o2) {
          return ((Comparable<Object>) o1).compareTo(o2);
        }
      };

  }

  protected abstract Set<AnEnum> create(AnEnum[] elements);

  @Override
  public AnEnum[] createArray(int length) {
    return new AnEnum[length];
  }

  /** Sorts the enums according to their natural ordering. */
  @Override
  public List<AnEnum> order(List<AnEnum> insertionOrder) {
    Collections.sort(insertionOrder);
    return insertionOrder;
  }

   *     indicates that the elements' <i>natural ordering</i> should be used.
   */
  @SuppressWarnings("unchecked")
  public static <E extends @Nullable Object> TreeMultiset<E> create(
      @CheckForNull Comparator<? super E> comparator) {
    return (comparator == null)
        ? new TreeMultiset<E>((Comparator) Ordering.natural())
        : new TreeMultiset<E>(comparator);
  }

  /**

@ElementTypesAreNonnullByDefault
final class RegularImmutableSortedSet<E> extends ImmutableSortedSet<E> {
  static final RegularImmutableSortedSet<Comparable> NATURAL_EMPTY_SET =
      new RegularImmutableSortedSet<>(ImmutableList.<Comparable>of(), Ordering.natural());

  @VisibleForTesting final transient ImmutableList<E> elements;

  RegularImmutableSortedSet(ImmutableList<E> elements, Comparator<? super E> comparator) {