anchor.diff:    }
   
     num get width => _width;
  +
     num get height => _height;
   
     operator [](int index) => _data[index];
  
positive.code: class Grid {
    int _width;
    int _height;
  
negative.code:     if (i < 0 || j < 0 || i >= _width || j >= _height) return -1;
      return _data[i][j];
    }
  
    void makePath(int sx, int sy, int ex, int ey, int tile) {
      set(sx, sy, tile);
      int nextpath;
  
      while (sx != ex || sy != ey) {
        if (sx != ex) {
          nextpath = RND(8);
          if (sx + nextpath > ex) {
            nextpath = ex - sx;
          }
          for (int i = 0; i < (nextpath); i++) {
            sx += 1;
            set(sx, sy, tile);
          }
        }
  
        if (sy != ey) {
          nextpath = RND(8);
          if (sy + nextpath > ey) {
            nextpath = ey - sy;
          }
          for (int i = 0; i < nextpath; i++) {
            sy += 1;
            set(sx, sy, tile);
          }
        }
      } //while
    }
  } //class
  