private void UnsubscribeFromPageEvents(ISequenceChangingNotifier page)
 {
     if (!_pages.Contains(page))
     {
         return;
     }
     page.Next     -= PageOnNext;
     page.Previous -= PageOnPrevious;
     page.PageReplacementRequested -= PageOnPageReplacementRequested;
 }
 private void SubscribeToPageEvents(ISequenceChangingNotifier page)
 {
     if (_pages.Contains(page))
     {
         return;
     }
     page.Next     += PageOnNext;
     page.Previous += PageOnPrevious;
     page.PageReplacementRequested += PageOnPageReplacementRequested;
 }