return (key == null) ? null : Maps.safeGet(delegate, key);
  }

  @Override
  ImmutableSet<Entry<K, V>> createEntrySet() {
    return ImmutableSet.unsafeDelegate(
        new ForwardingSet<Entry<K, V>>() {
          @Override
          protected Set<Entry<K, V>> delegate() {
            return delegate.entrySet();
          }

          @Override

            CollectionFeature.ALLOWS_NULL_VALUES,
            CollectionSize.ANY,
            CollectionFeature.SERIALIZABLE)
        .createTestSuite();
  }

  static class TestSet<E> extends ForwardingSet<E> implements Serializable {
    final Set<E> delegate;
    public Object mutex;

    public TestSet(Set<E> delegate, @Nullable Object mutex) {
      this.delegate = delegate;
      this.mutex = mutex;
    }

            CollectionFeature.ALLOWS_NULL_VALUES,
            CollectionSize.ANY,
            CollectionFeature.SERIALIZABLE)
        .createTestSuite();
  }

  static class TestSet<E> extends ForwardingSet<E> implements Serializable {
    final Set<E> delegate;
    public final Object mutex;

    public TestSet(Set<E> delegate, Object mutex) {
      checkNotNull(mutex);
      this.delegate = delegate;

      this.delegate = delegate;
    }

    @Override
    protected Map<K, V> delegate() {
      return delegate;
    }

    @Override
    public Set<K> keySet() {
      return new ForwardingSet<K>() {
        @Override
        protected Set<K> delegate() {
          return delegate.keySet();
        }

        @Override
        public Iterator<K> iterator() {

  /**
   * Returns the backing delegate instance that methods are forwarded to. Abstract subclasses
   * generally override this method with an abstract method that has a more specific return type,
   * such as {@link ForwardingSet#delegate}. Concrete subclasses override this method to supply the
   * instance being decorated.
   */
  protected abstract Object delegate();

 *
 * @author Mike Bostock
 * @author Louis Wasserman
 * @since 2.0
 */
@GwtCompatible
@ElementTypesAreNonnullByDefault
public abstract class ForwardingSortedSet<E extends @Nullable Object> extends ForwardingSet<E>
    implements SortedSet<E> {

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

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

  @Override
  @CheckForNull

 *
 * @author Mike Bostock
 * @author Louis Wasserman
 * @since 2.0
 */
@GwtCompatible
@ElementTypesAreNonnullByDefault
public abstract class ForwardingSortedSet<E extends @Nullable Object> extends ForwardingSet<E>
    implements SortedSet<E> {

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

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

  @Override
  @CheckForNull

                      @Override
                      public Set<Entry<String>> entrySet() {
                        final Set<Entry<String>> backingSet = super.entrySet();
                        return new ForwardingSet<Entry<String>>() {
                          @Override
                          protected Set<Entry<String>> delegate() {
                            return backingSet;
                          }

                      @Override
                      public Set<Entry<String>> entrySet() {
                        final Set<Entry<String>> backingSet = super.entrySet();
                        return new ForwardingSet<Entry<String>>() {
                          @Override
                          protected Set<Entry<String>> delegate() {
                            return backingSet;
                          }

      return new AddRejectingSet<>(key);
    } else {
      return new AddRejectingList<>(key);
    }
  }

  static class AddRejectingSet<K extends @Nullable Object, V extends @Nullable Object>
      extends ForwardingSet<V> {
    @ParametricNullness final K key;

    AddRejectingSet(@ParametricNullness K key) {
      this.key = key;
    }

    @Override
    public boolean add(@ParametricNullness V element) {