inner.mutex = outer;
    outer.add("foo"); // necessary because we try to remove elements later on
    return outer;
  }

  private static final class TestQueue<E> implements Queue<E> {
    private final Queue<E> delegate = Lists.newLinkedList();
    public Object mutex;

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

      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));

  private Object mutex() {
    Object mutex = mutexDoNotUseDirectly;
    if (mutex == null) {
      synchronized (this) {
        mutex = mutexDoNotUseDirectly;
        if (mutex == null) {
          mutexDoNotUseDirectly = mutex = new Object();
        }
      }
    }
    return mutex;
  }

  RateLimiter(SleepingStopwatch stopwatch) {

  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.

    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;

      checkNotNull(mutex);
      this.delegate = delegate;
      this.mutex = mutex;
    }

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

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

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

    final Table<R, C, V> delegate = HashBasedTable.create();
    public final Object mutex = new Integer(1); // something Serializable

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

    @Override
    public boolean equals(@Nullable Object o) {
      assertTrue(Thread.holdsLock(mutex));
      return delegate.equals(o);
    }

    @Override

    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();
    }