anchor.diff:  
       // Add an explosion
       addExplosion(AsteroidSize.large, _ship.position);
  -
  -    _navigator.pop();
     }
   
     // Handling controls
  
positive.code: 
      // Set game over
      _isGameOver = true;
      _gameOverCallback(_hud.score);
  
      // Remove the ship
  
negative.code:   // Game state
    int _numFrames = 0;
    bool _isGameOver = false;
    int _gameOverFrame;
    int _currentLevel = 0;
  
    // Heads up display
  