anchor.diff:  
   import 'package:flutter/material.dart';
   
  +final List<Book> books = [
  +  Book('Stranger in a Strange Land', Author('Robert A. Heinlein')),
  +  Book('Foundation', Author('Isaac Asimov')),
  +  Book('Fahrenheit 451', Author('Ray Bradbury')),
  +];
  +
  +List<Author> get authors => [
  +  ...books.map((book) => book.author),
  +];
  +
   void main() {
     runApp(BooksApp());
   }
  
positive.code:           MaterialPage(
              key: ValueKey('AuthorsListPage'),
              child: AuthorsListScreen(
                authors: _appState.authors,
                onTapped: _handleAuthorTapped,
                onGoToBooksTapped: _handleGoToBooksTapped,
              ),
  
negative.code:           MaterialPage(
              key: ValueKey('BooksListPage'),
              child: BooksListScreen(
                books: books,
                onTapped: _handleBookTapped,
              ),
            ),
  