anchor.diff:  }
   
   class AppState extends ChangeNotifier {
  +  final List<Book> _books = [
  +    Book('Stranger in a Strange Land', 'Robert A. Heinlein'),
  +    Book('Foundation', 'Isaac Asimov'),
  +    Book('Fahrenheit 451', 'Ray Bradbury'),
  +  ];
  +
     int? _selectedBookId;
   
     int? get selectedBookId => _selectedBookId;
  
positive.code:         MaterialPage(
            key: ValueKey('BooksListPage'),
            child: BooksListScreen(
              books: books,
              onTapped: _handleBookTapped,
            ),
          ),
  
negative.code: // for details. All rights reserved. Use of this source code is governed by a
  // BSD-style license that can be found in the LICENSE file.
  
  import 'package:flutter/material.dart';
  
  void main() {
    runApp(BooksApp());
  