anchor.diff:      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++;
     }
   
  
positive.code:   // Game state
    int _numFrames = 0;
    bool _isGameOver = false;
    int _currentLevel = 0;
  
    // Heads up display
  
negative.code: 
      // Add an explosion
      addExplosion(AsteroidSize.large, _ship.position);
    }
  
    // Handling controls
  