}

  static <E extends @Nullable Object> Deque<E> deque(Deque<E> deque, @CheckForNull 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, @CheckForNull Object mutex) {
      super(delegate, mutex);
    }

  }

  static <E extends @Nullable Object> Deque<E> deque(Deque<E> deque, @CheckForNull 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, @CheckForNull Object mutex) {
      super(delegate, mutex);
    }

    private static class ActivationWalker extends MavenTransformer {

        private final Deque<ActivationFrame> stk;

        ActivationWalker(Deque<ActivationFrame> stk, UnaryOperator<String> transformer) {
            super(transformer);
            this.stk = stk;
        }

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

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

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