* visit elements during a traversal once and only once.
   */
  @VisibleForTesting
  @CanIgnoreReturnValue
  @CheckForNull
  MoveDesc<E> removeAt(int index) {
    checkPositionIndex(index, size);
    modCount++;
    size--;
    if (size == index) {
      queue[size] = null;
      return null;
    }
    E actualLastElement = elementData(size);