public Collision collision( GameObject with )
  {
    if ( topX >= with.getX()+with.getWidth()  ||
         topX+width <= with.getX()            ||
         topY >= with.getY()+with.getHeight() ||
         topY+height <= with.getY() )
      return Collision.NO_HIT;
    else {
      return Collision.HIT;
    }
  }