}

  static <E extends @Nullable Object> Deque<E> deque(Deque<E> deque, @Nullable Object mutex) {
    return new SynchronizedDeque<>(deque, mutex);
  }

  static final class SynchronizedDeque<E extends @Nullable Object> extends SynchronizedQueue<E>
      implements Deque<E> {

    SynchronizedDeque(Deque<E> delegate, @Nullable Object mutex) {
      super(delegate, mutex);
    }

  }

  static <E extends @Nullable Object> Deque<E> deque(Deque<E> deque, @Nullable Object mutex) {
    return new SynchronizedDeque<>(deque, mutex);
  }

  static final class SynchronizedDeque<E extends @Nullable Object> extends SynchronizedQueue<E>
      implements Deque<E> {

    SynchronizedDeque(Deque<E> delegate, @Nullable Object mutex) {
      super(delegate, mutex);
    }

    }

    /**
     * Creates and returns a new instance of {@link ArrayDeque}.
     *
     * @param <E> the element type of {@link ArrayDeque}
     * @param c the collection of elements to be placed in the deque
     * @return a new instance of {@link ArrayDeque}
     * @see ArrayDeque#ArrayDeque(Collection)
     */
    public static <E> ArrayDeque<E> newArrayDeque(final Collection<? extends E> c) {

     * meta-iterators rather than letting the nesting get arbitrarily deep.  This keeps each
     * operation O(1).
     */

    private @Nullable Iterator<? extends Iterator<? extends T>> topMetaIterator;

    // Only becomes nonnull if we encounter nested concatenations.
    private @Nullable Deque<Iterator<? extends Iterator<? extends T>>> metaIterators;