anchor.diff:  
       // Add an explosion
       addExplosion(AsteroidSize.large, _ship.position);
  -
  -    _navigator.pop();
     }
   
     // Handling controls
  
positive.code:   // Game state
    int _numFrames = 0;
    bool _isGameOver = false;
    int _currentLevel = 0;
  
    // Heads up display
  
negative.code:     if (_numFrames > _numFramesShieldActive) _shield.visible = false;
      else if (_numFrames > _numFramesShieldActive - _numFramesShieldFlickers) _shield.visible = !_shield.visible;
  
      // Check for exit back to main screen
      if (_isGameOver && _numFrames - _gameOverFrame == 60) {
        _navigator.pop();
      }
  
      _numFrames++;
    }
  
  