* is now at some position before {@code index}. The second element is the one that was swapped
   * down to replace the element at {@code index}. This fact is used by iterator.remove so as to
   * visit elements during a traversal once and only once.
   */
  @VisibleForTesting
  @CanIgnoreReturnValue
  @CheckForNull
  MoveDesc<E> removeAt(int index) {
    checkPositionIndex(index, size);
    modCount++;