anchor.diff:    Array2D<Actor> _actor;
   
     Stage(Size size) {
  -    _terrain = new Array2D<Terrain>(size, const Terrain('.', 'silver'));
  +    _terrain = new Array2D<Terrain>(size, TerrainTable.get('.'));
       _actor = new Array2D<Actor>(size);
     }
   
  
positive.code:   }
  
    void setTerrain(String symbol, Coordinate coordinate) {
      _terrain[coordinate] = TerrainDB.get(symbol);
    }
  }
  
  
negative.code:   void operator []=(Coordinate pos, T value) {
      _elements[_index(pos)] = value;
    }
    /// indexから座標を返す
    Coordinate _coordinate(int index) => new Coordinate(index % size.width, index ~/ size.width);
    /// 座標から、実際のindexを返す
  