TestMap<K, V> inner = new TestMap<>(innermost, mutex);
    NavigableMap<K, V> outer = Synchronized.navigableMap(inner, mutex);
    return outer;
  }

  static class TestEntry<K, V> extends ForwardingMapEntry<K, V> implements Serializable {
    private final Entry<K, V> delegate;
    private final Object mutex;

    TestEntry(Entry<K, V> delegate, Object mutex) {
      this.delegate = delegate;
      this.mutex = mutex;

    final Set<E> delegate;
    public Object mutex;

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

    @Override
    protected Set<E> delegate() {
      return delegate;
    }

    @Override
    public String toString() {
      assertTrue(Thread.holdsLock(mutex));
      return super.toString();
    }

    @Override

    public TestBiMap(BiMap<K, V> delegate, Object mutex) {
      super(delegate, mutex);
      this.delegate = delegate;
    }

    @Override
    public @Nullable V forcePut(K key, V value) {
      assertTrue(Thread.holdsLock(mutex));
      return delegate.forcePut(key, value);
    }

    @Override
    public BiMap<V, K> inverse() {
      assertTrue(Thread.holdsLock(mutex));
      return delegate.inverse();
    }

      return delegate.offer(o);
    }

    @Override
    public @Nullable E poll() {
      assertTrue(Thread.holdsLock(mutex));
      return delegate.poll();
    }

    @Override
    public E remove() {
      assertTrue(Thread.holdsLock(mutex));
      return delegate.remove();
    }

    @Override
    public boolean remove(Object object) {
      assertTrue(Thread.holdsLock(mutex));

      return delegate.offer(o);
    }

    @Override
    public @Nullable E poll() {
      assertTrue(Thread.holdsLock(mutex));
      return delegate.poll();
    }

    @Override
    public E remove() {
      assertTrue(Thread.holdsLock(mutex));
      return delegate.remove();
    }

    @Override
    public boolean remove(Object object) {
      assertTrue(Thread.holdsLock(mutex));

    assertSame(MUTEX, ((SynchronizedNavigableSet<String>) descendingSet).mutex);
  }

  public void testHeadSet_E() {
    NavigableSet<String> map = create();
    SortedSet<String> headSet = map.headSet("a");
    assertTrue(headSet instanceof SynchronizedSortedSet);
    assertSame(MUTEX, ((SynchronizedSortedSet<String>) headSet).mutex);
  }

  public void testHeadSet_E_B() {

    NavigableSet<E> outer = Synchronized.navigableSet(inner, null);
    inner.mutex = outer;
    return outer;
  }

  static class TestSet<E> extends SynchronizedSetTest.TestSet<E> implements NavigableSet<E> {

    TestSet(NavigableSet<E> delegate, @Nullable Object mutex) {
      super(delegate, mutex);
    }

    @Override
    protected NavigableSet<E> delegate() {

  private Synchronized() {}

  static class SynchronizedObject implements Serializable {
    final Object delegate;
    final Object mutex;

    SynchronizedObject(Object delegate, @CheckForNull Object mutex) {
      this.delegate = checkNotNull(delegate);
      this.mutex = (mutex == null) ? this : mutex;
    }

    Object delegate() {
      return delegate;
    }

    // No equals and hashCode; see ForwardingObject for details.

      return super.equals(o);
    }

    @Override
    public int hashCode() {
      assertTrue(Thread.holdsLock(mutex));
      return super.hashCode();
    }

    @Override
    public int size() {
      assertTrue(Thread.holdsLock(mutex));
      return super.size();
    }

    @Override
    public boolean isEmpty() {
      assertTrue(Thread.holdsLock(mutex));

    TestMap<K, V> inner = new TestMap<>(innermost, mutex);
    NavigableMap<K, V> outer = Synchronized.navigableMap(inner, mutex);
    return outer;
  }

  static class TestEntry<K, V> extends ForwardingMapEntry<K, V> implements Serializable {
    private final Entry<K, V> delegate;
    private final Object mutex;

    TestEntry(Entry<K, V> delegate, Object mutex) {
      this.delegate = delegate;
      this.mutex = mutex;