anchor.diff:    void operator []=(Coordinate pos, T value) {
       _elements[_index(pos)] = value;
     }
  -  /// 指定値で埋める
  -  void fill(T value) => _elements.fillRange(0, _elements.length, 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
  
positive.code:   Array2D<Actor> _actor;
  
    Stage(Size size) {
      _terrain = new Array2D<Terrain>(size, const Terrain('.', 'silver'));
      _actor = new Array2D<Actor>(size);
    }
  
  
negative.code:   }
  
    void setTerrain(String symbol, Coordinate coordinate) {
      _terrain[coordinate] = TerrainTable.get(symbol);
    }
  }
  
  