public E higher(E e) {
      synchronized (mutex) {
        return delegate().higher(e);
      }
    }

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

    @Override
    @CheckForNull
    public E pollFirst() {
      synchronized (mutex) {
        return delegate().pollFirst();