anchor.diff:  
     void _incrementCounter() {
       setState(() {
  -      // This call to setState tells the Flutter framework that something has
  -      // changed in this State, which causes it to rerun the build method below
  -      // so that the display can reflect the updated values. If we changed
  -      // _counter without calling setState(), then the build method would not be
  -      // called again, and so nothing would appear to happen.
         _counter++;
       });
     }
   
     @override
     Widget build(BuildContext context) {
  -    // This method is rerun every time setState is called, for instance as done
  -    // by the _incrementCounter method above.
  -    //
  -    // The Flutter framework has been optimized to make rerunning build methods
  -    // fast, so that you can just rebuild anything that needs updating rather
  -    // than having to individually change instances of widgets.
       return Scaffold(
         appBar: AppBar(
  -        // Here we take the value from the MyHomePage object that was created by
  -        // the App.build method, and use it to set our appbar title.
           title: Text(widget.title),
         ),
         body: Center(
  -        // Center is a layout widget. It takes a single child and positions it
  -        // in the middle of the parent.
           child: Column(
  -          // Column is also a layout widget. It takes a list of children and
  -          // arranges them vertically. By default, it sizes itself to fit its
  -          // children horizontally, and tries to be as tall as its parent.
  -          //
  -          // Invoke "debug painting" (press "p" in the console, choose the
  -          // "Toggle Debug Paint" action from the Flutter Inspector in Android
  -          // Studio, or the "Toggle Debug Paint" command in Visual Studio Code)
  -          // to see the wireframe for each widget.
  -          //
  -          // Column has various properties to control how it sizes itself and
  -          // how it positions its children. Here we use mainAxisAlignment to
  -          // center the children vertically; the main axis here is the vertical
  -          // axis because Columns are vertical (the cross axis would be
  -          // horizontal).
             mainAxisAlignment: MainAxisAlignment.center,
             children: <Widget>[
               Text(
  
positive.code:     return MaterialApp(
        title: 'Flutter Demo',
        theme: ThemeData(
          // This is the theme of your application.
          //
          // Try running your application with "flutter run". You'll see the
          // application has a blue toolbar. Then, without quitting the app, try
          // changing the primarySwatch below to Colors.green and then invoke
          // "hot reload" (press "r" in the console where you ran "flutter run",
          // or simply save your changes to "hot reload" in a Flutter IDE).
          // Notice that the counter didn't reset back to zero; the application
          // is not restarted.
          primarySwatch: Colors.blue,
          // This makes the visual density adapt to the platform that you run
          // the app on. For desktop platforms, the controls will be smaller and
          // closer together (more dense) than on mobile platforms.
          visualDensity: VisualDensity.adaptivePlatformDensity,
        ),
        home: MyHomePage(title: 'Flutter Demo Home Page'),
  
negative.code:         onPressed: _incrementCounter,
          tooltip: 'Increment',
          child: Icon(Icons.add),
        ),
      );
    }
  }
  