@Override
    @CheckForNull
    public E floor(@ParametricNullness E e) {
      return delegate.floor(e);
    }

    @Override
    @CheckForNull
    public E ceiling(@ParametricNullness E e) {
      return delegate.ceiling(e);
    }

    @Override
    @CheckForNull
    public E higher(@ParametricNullness E e) {
      return delegate.higher(e);
    }

    @Override

    @Override
    NavigableSet<E> delegate() {
      return (NavigableSet<E>) super.delegate();
    }

    @Override
    @CheckForNull
    public E ceiling(E e) {
      synchronized (mutex) {
        return delegate().ceiling(e);
      }
    }

    @Override
    public Iterator<E> descendingIterator() {
      return delegate().descendingIterator(); // manually synchronized
    }

    @Override
    NavigableSet<E> delegate() {
      return (NavigableSet<E>) super.delegate();
    }

    @Override
    @CheckForNull
    public E ceiling(E e) {
      synchronized (mutex) {
        return delegate().ceiling(e);
      }
    }

    @Override
    public Iterator<E> descendingIterator() {
      return delegate().descendingIterator(); // manually synchronized
    }