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

    final Set<E> delegate;
    public final Object mutex;

    public TestSet(Set<E> delegate, Object mutex) {
      checkNotNull(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();
    }