*
 * @author Louis Wasserman
 * @since 12.0
 */
@GwtIncompatible
public abstract class ForwardingNavigableSet<E extends @Nullable Object>
    extends ForwardingSortedSet<E> implements NavigableSet<E> {

  /** Constructor for use by subclasses. */
  protected ForwardingNavigableSet() {}

  @Override
  protected abstract NavigableSet<E> delegate();

  @Override

 *
 * @author Louis Wasserman
 * @since 12.0
 */
@GwtIncompatible
public abstract class ForwardingNavigableSet<E extends @Nullable Object>
    extends ForwardingSortedSet<E> implements NavigableSet<E> {

  /** Constructor for use by subclasses. */
  protected ForwardingNavigableSet() {}

  @Override
  protected abstract NavigableSet<E> delegate();

  @Override

      return set;
    }
    return new UnmodifiableNavigableSet<>(set);
  }

  static final class UnmodifiableNavigableSet<E extends @Nullable Object>
      extends ForwardingSortedSet<E> implements NavigableSet<E>, Serializable {
    private final NavigableSet<E> delegate;
    private final SortedSet<E> unmodifiableDelegate;

    UnmodifiableNavigableSet(NavigableSet<E> delegate) {