/** Constructor for use by subclasses. */
  protected ForwardingDeque() {}

  @Override
  protected abstract Deque<E> delegate();

  @Override
  public void addFirst(@ParametricNullness E e) {
    delegate().addFirst(e);
  }

  @Override
  public void addLast(@ParametricNullness E e) {
    delegate().addLast(e);
  }

  @Override
  public Iterator<E> descendingIterator() {

  /** Constructor for use by subclasses. */
  protected ForwardingDeque() {}

  @Override
  protected abstract Deque<E> delegate();

  @Override
  public void addFirst(@ParametricNullness E e) {
    delegate().addFirst(e);
  }

  @Override
  public void addLast(@ParametricNullness E e) {
    delegate().addLast(e);
  }

  @Override
  public Iterator<E> descendingIterator() {

    public <T> T[] toArray(T[] array) {
      assertTrue(Thread.holdsLock(mutex));
      return delegate.toArray(array);
    }

    @Override
    public void addFirst(E e) {
      assertTrue(Thread.holdsLock(mutex));
      delegate.addFirst(e);
    }

    @Override
    public void addLast(E e) {
      assertTrue(Thread.holdsLock(mutex));
      delegate.addLast(e);
    }

    @Override

     * </p>
     *
     * @param disposable
     *            A disposable resource. Must not be {@literal null}.
     */
    public static synchronized void addFirst(final Disposable disposable) {
        assertArgumentNotNull("disposable", disposable);
        disposables.addFirst(disposable);
    }

    /**
     * Unregisters a disposable resource.
     *
     * @param disposable

    public <T> T[] toArray(T[] array) {
      assertTrue(Thread.holdsLock(mutex));
      return delegate.toArray(array);
    }

    @Override
    public void addFirst(E e) {
      assertTrue(Thread.holdsLock(mutex));
      delegate.addFirst(e);
    }

    @Override
    public void addLast(E e) {
      assertTrue(Thread.holdsLock(mutex));
      delegate.addLast(e);
    }

    @Override

    abstract @Nullable N visitNext(Deque<Iterator<? extends N>> horizon);
  }

  /** Poor man's method reference for {@code Deque::addFirst} and {@code Deque::addLast}. */
  private enum InsertionOrder {
    FRONT {
      @Override
      <T> void insertInto(Deque<T> deque, T value) {
        deque.addFirst(value);
      }
    },
    BACK {
      @Override
      <T> void insertInto(Deque<T> deque, T value) {

        header.previous.remove();
        return last;
    }

    /**
     * Adds an element at the beginning.
     *
     * @param element the object to be added
     */
    public void addFirst(final E element) {
        header.next.addBefore(element);
    }

    /**
     * Adds an element at the end.
     *
     * @param element the object to be added
     */

        assertThat(list.getLast(), is("1"));
    }

    /**
     * @throws Exception
     */
    @Test
    public void testAddFirst() throws Exception {
        list.addFirst("1");
        list.addFirst("2");
        assertThat(list.getFirst(), is("2"));
    }

    /**
     * @throws Exception
     */
    @Test
    public void testAddLast() throws Exception {

    }

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

    @Override
    public void addFirst(E e) {
      synchronized (mutex) {
        delegate().addFirst(e);
      }
    }

    @Override
    public void addLast(E e) {
      synchronized (mutex) {
        delegate().addLast(e);
      }
    }

    }

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

    @Override
    public void addFirst(E e) {
      synchronized (mutex) {
        delegate().addFirst(e);
      }
    }

    @Override
    public void addLast(E e) {
      synchronized (mutex) {
        delegate().addLast(e);
      }
    }