anchor.diff:  
     @override
     Future<void> setNewRoutePath(BookRoutePath path) async {
  -    filter = path.filter;
  +    _appState.filter = path.filter;
     }
   }
   
  
positive.code:         MaterialPage(
            key: ValueKey('BooksListPage'),
            child: BooksListScreen(
              books: books,
              filter: filter,
              onFilterChanged: (filter) {
                this.filter = filter;
                notifyListeners();
              },
            ),
          ),
  
negative.code:   final String author;
  
    Book(this.title, this.author);
  }
  
  class AppState extends ChangeNotifier {
    List<Book> books = [
      Book('Stranger in a Strange Land', 'Robert A. Heinlein'),
      Book('Foundation', 'Isaac Asimov'),
      Book('Fahrenheit 451', 'Ray Bradbury'),
    ];
    String? _filter;
  
    String? get filter => _filter;
  
    set filter(String? filter) {
      _filter = filter;
      notifyListeners();
    }
  }
  
  class BooksApp extends StatefulWidget {
  