}

    @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);
      }
    }

     * @param collection the sequenced collection
     * @param element the element to add
     * @since Java 21
     */
    public static <E> void addFirst(final SequencedCollection<E> collection, final E element) {
        collection.addFirst(element);
    }

    /**
     * Adds an element to the end of a sequenced collection.
     *
     * @param <E> the element type

            this.metaIterators = new ArrayDeque<>();
          }
          this.metaIterators.addFirst(this.topMetaIterator);
          if (topConcat.metaIterators != null) {
            while (!topConcat.metaIterators.isEmpty()) {
              this.metaIterators.addFirst(topConcat.metaIterators.removeLast());
            }
          }
          this.topMetaIterator = topConcat.topMetaIterator;

            this.metaIterators = new ArrayDeque<>();
          }
          this.metaIterators.addFirst(this.topMetaIterator);
          if (topConcat.metaIterators != null) {
            while (!topConcat.metaIterators.isEmpty()) {
              this.metaIterators.addFirst(topConcat.metaIterators.removeLast());
            }
          }
          this.topMetaIterator = topConcat.topMetaIterator;