anchor.diff:    int get currentposition => _currentposition;
     set currentposition(int n) => _currentposition = n;
   
  -  int currentposforward() => _currentposition++;
  +  int currentposforward() => this._currentposition++;
   
     int currentposbackward() => _currentposition--;
   
  
positive.code:   var input = querySelector('#tarea');
    input.onKeyUp.listen((e) {
      if (tr.currentposition < target.length) {
        tr.handleKeyEvent(e);
      }
    });
  
negative.code:     var sofar = targettext.split('').getRange(0, _currentposition + 1);
      return 100 - (_errors.toDouble() / sofar.length.toDouble() * 100).round();
    }
    void handleKeyEvent(Event e) {  
      KeyEvent k = new KeyEvent.wrap(e);
      var tinput = querySelector('#tarea');
      switch (k.keyCode) {
        case KeyCode.SHIFT:
          break;
           case KeyCode.BACKSPACE:
          if (this.currentposition > 0) {
            this.currentposbackward();
            break;
  