@Override
  @ParametricNullness
  public E removeFirst() {
    return delegate().removeFirst();
  }

  @CanIgnoreReturnValue
  @Override
  @ParametricNullness
  public E removeLast() {
    return delegate().removeLast();
  }

  @CanIgnoreReturnValue
  @Override
  public boolean removeFirstOccurrence(@Nullable Object o) {
    return delegate().removeFirstOccurrence(o);
  }

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

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

    @Override
    public @Nullable E pollFirst() {
      assertTrue(Thread.holdsLock(mutex));
      return delegate.pollFirst();
    }

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

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

    @Override
    public @Nullable E pollFirst() {
      assertTrue(Thread.holdsLock(mutex));
      return delegate.pollFirst();
    }

    }

    @Override
    public T next() {
      Iterator<T> itr = stack.getLast(); // throws NSEE if empty
      T result = checkNotNull(itr.next());
      if (!itr.hasNext()) {
        stack.removeLast();
      }
      Iterator<T> childItr = children(result).iterator();
      if (childItr.hasNext()) {
        stack.addLast(childItr);
      }
      return result;
    }
  }

  /**

    }

    @Override
    public T next() {
      Iterator<T> itr = stack.getLast(); // throws NSEE if empty
      T result = checkNotNull(itr.next());
      if (!itr.hasNext()) {
        stack.removeLast();
      }
      Iterator<T> childItr = children(result).iterator();
      if (childItr.hasNext()) {
        stack.addLast(childItr);
      }
      return result;
    }
  }

  /**

    }

    /**
     * Disposes of all registered resources.
     */
    public static synchronized void dispose() {
        while (!disposables.isEmpty()) {
            final Disposable disposable = disposables.removeLast();
            try {
                disposable.dispose();
            } catch (final Throwable t) {
                t.printStackTrace(); // must not use Logger.
            }
        }
        disposables.clear();

    stack.addLast(new NodeAndRemainingSuccessors<>(startNode));

    while (!stack.isEmpty()) {
      // To peek at the top two items, we need to temporarily remove one.
      NodeAndRemainingSuccessors<N> top = stack.removeLast();
      NodeAndRemainingSuccessors<N> prev = stack.peekLast();
      stack.addLast(top);

      N node = top.node;
      N previousNode = prev == null ? null : prev.node;

    stack.addLast(new NodeAndRemainingSuccessors<>(startNode));

    while (!stack.isEmpty()) {
      // To peek at the top two items, we need to temporarily remove one.
      NodeAndRemainingSuccessors<N> top = stack.removeLast();
      NodeAndRemainingSuccessors<N> prev = stack.peekLast();
      stack.addLast(top);

      N node = top.node;
      N previousNode = prev == null ? null : prev.node;

    }

    /**
     * @throws Exception
     */
    @Test
    public void testRemoveLast() throws Exception {
        list.addLast("1");
        list.addLast("2");
        list.removeLast();
        assertThat(list.getLast(), is("1"));
    }

    /**
     * @throws Exception
     */
    @Test
    public void testAddFirst() throws Exception {
        list.addFirst("1");

        header.next.remove();
        return first;
    }

    /**
     * Removes the last element.
     *
     * @return the last element
     */
    public E removeLast() {
        if (isEmpty()) {
            throw new NoSuchElementException();
        }
        final E last = header.previous.element;
        header.previous.remove();
        return last;
    }