*
 * @author Louis Wasserman
 * @since 12.0
 */
@GwtIncompatible
@ElementTypesAreNonnullByDefault
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
  @CheckForNull

import junit.framework.TestSuite;
import org.checkerframework.checker.nullness.qual.Nullable;

/**
 * Tests for {@code ForwardingNavigableSet}.
 *
 * @author Louis Wasserman
 */
public class ForwardingNavigableSetTest extends TestCase {
  static class StandardImplForwardingNavigableSet<T> extends ForwardingNavigableSet<T> {
    private final NavigableSet<T> backingSet;

    StandardImplForwardingNavigableSet(NavigableSet<T> backingSet) {