anchor.diff:      if (idx == null) {
         return null;
       }
  -    return books[idx];
  -  }
  -
  -  set selectedBook(Book? book) {
  -    if (book == null) {
  -      _selectedBookId = null;
  -      notifyListeners();
  -      return;
  -    }
  -
  -    var id = books.indexOf(book);
  -    if (id == -1) {
  -      _selectedBookId = null;
  -    } else {
  -      _selectedBookId = id;
  -    }
  -
  -    notifyListeners();
  -  }
  +    return _books[idx];
  +  }
  +
  +  List<Book> get allBooks => _books;
   
     void clearSelectedBook() {
       _selectedBookId = null;
  
positive.code: }
  
  class AppState extends ChangeNotifier {
    int? _selectedBookId;
  
    int? get selectedBookId => _selectedBookId;
  
negative.code:         MaterialPage(
            key: ValueKey('BooksListPage'),
            child: BooksListScreen(
              books: _appState.allBooks,
              onTapped: _handleBookTapped,
            ),
          ),
  