anchor.diff:  }
   
   abstract class Authentication {
  -  bool isSignedIn();
  -
  -  void signOut();
  +  Future<bool> isSignedIn();
  +
  +  Future<void> signOut();
   
     Future<bool> signIn(String username, String password);
   }
  
positive.code:   }
  
    @override
    Widget build(BuildContext context) {
      final isSignedIn = _appState.auth.isSignedIn();
      final viewingBooksScreen = _appState.viewingBooks;
      return Navigator(
        key: navigatorKey,
        pages: [
          if (isSignedIn) ...[
            MaterialPage(
              key: ValueKey('HomeScreen'),
              child: HomeScreen(
                onGoToBooks: _handleGoToBooks,
              ),
            ),
            if (viewingBooksScreen)
  
negative.code:               key: ValueKey('BooksListPage'),
                child: BooksListScreen(),
              ),
          ]
        ],
        onPopPage: (route, result) {
          if (!route.didPop(result)) {
            return false;
          }
  
          if (_appState.isViewingBooks) {
            _appState.isViewingBooks = false;
          }
  
          return true;
        },
      );
    }
  }
  
  class AppRoutePath {}
  