anchor.diff:          MaterialPage(
             key: ValueKey('BooksListPage'),
             child: BooksListScreen(
  -            books: books,
  -            filter: filter,
  +            books: _appState.books,
  +            filter: _appState.filter,
               onFilterChanged: (filter) {
  -              this.filter = filter;
  -              notifyListeners();
  +              _appState.filter = filter;
               },
             ),
           ),
  
positive.code:   final String author;
  
    Book(this.title, this.author);
  }
  
  class BooksApp extends StatefulWidget {
  
negative.code: 
    @override
    Future<void> setNewRoutePath(BookRoutePath path) async {
      _appState.filter = path.filter;
    }
  }
  
  