code: public void set(int x, int y) {
  		if (x<0 || y<0) {
  			x=-1;
  			y=-1;
  		}
  		if (this.x!=x || this.y!=y) {
  			this.x=x;
  			this.y=y;
  			fireChange();
  		}
  	}
docs: Changes the pixel stored in this model. If either coordinate is negative, the new value of the model will be
  (-1, -1).
  @param x the new x coordinate
  @param y the new y coordinate