anchor.diff:      Grid grid;
       num xsize, ysize;
       CanvasRenderingContext2D ctx;
  -    var hlSquare = null; // current highlighted square
  +    GridPoint hlSquare = null; // current highlighted square
   
       num stepSizeX, stepSizeY;
       num boxDimX, boxDimY;
  
positive.code:         return level == 0;
      }
  
      String toString() {
          return level.toString();
      }
  }
  
  class Grid {
      final int x, y;
      List<Elem> elems;
  
  
negative.code:     });
  
      canvas.onMouseDown.listen( (e) {
          dg.setPoint(e.client.x, e.client.y);
      });
  }
  