anchor.diff:    var tr = new Trainer(target);
     var input = querySelector('#tarea');
     input.onKeyUp.listen((e) {
  -    while (tr.currentposition < target.length) {
  +    if (tr.currentposition < target.length) {
         tr.handleKeyEvent(e);
       }
     });
  
positive.code:       case KeyCode.SHIFT:
          break;
        case KeyCode.BACKSPACE:
          if (_currentposition > 0) {
            currentposbackward();
            break;
          }
          break;
      }
    }
  }
  
negative.code: 
    int currentposbackward() => _currentposition--;
  
    String charAtPosition() => targettext.split('')[this._currentposition];
  
    set errors(int n) => _errors = n;
  
  