anchor.diff:  
     @override
     Widget build(BuildContext context) {
  -    return new Flexible(
  -      flex: _flex,
  -      child: new Row(
  -        children: <Widget>[
  -          new MainKeyPad(calcState: calcState),
  -          new OpKeyPad(calcState: calcState),
  -        ]
  +    return new Theme(
  +      data: new ThemeData(
  +        primarySwatch: Colors.purple,
  +        brightness: ThemeBrightness.dark
  +      ),
  +      child: new Flexible(
  +        flex: _flex,
  +        child: new Material(
  +          child: new Row(
  +            children: <Widget>[
  +              new MainKeyPad(calcState: calcState),
  +              new OpKeyPad(calcState: calcState),
  +            ]
  +          )
  +        )
         )
       );
     }
  
positive.code:       child: new Center(
          child: new Text(
            _contents,
            style: const TextStyle(color: Colors.black, fontSize: 24.0)
          )
        )
      );
  
negative.code:         children: <Widget>[
            // Give the key-pad 3/5 of the vertical space and the display 2/5.
            new CalcDisplay(2, _expression.toString()),
            new Divider(height: 1.0),
            new KeyPad(3, calcState: this)
          ]
        )
  