anchor.diff:        (+ (x (field-offset game)) (x (field-dim game)))
         (+ (y (field-offset game)) (y (field-dim game))))
       :color sdl:*green*)
  -  (draw (paddle game) (xform game)))
  +  (draw (paddle game) (xform game))
  +  (mapc (lambda (b) (draw b (xform game)))
  +        (balls game))
  +  )
   
   ;; (game-loop 'breaker)
  
positive.code: 
  (defclass paddle (color-rect) ())
  
  (defclass ball (obj-circle has-color has-velocity) ())
  
  (defmethod add ((obj1 vector-2d) (obj2 vector-2d))
    (vector-2d (+ (x obj1) (x obj2)) (+ (y obj1) (y obj2))))
  
negative.code:       (sdl:rectangle-from-edges-* (x loc) (y loc) (x corner) (y corner))
        :color (color obj))))
  
  (defmethod draw ((obj ball) (xform xform-2d))
    (let ((loc (apply-xform (location obj) xform))
          (rad (* (radius obj) (x (scale xform)))))
      (sdl:draw-filled-circle-* (x loc) (y loc) rad :color (color obj))))
  
  ;; parameters
  (defparameter *paddle-speed* 8)
  (defparameter *ball-radius* 5)
  (defparameter *ball-init-velocity* (vector-2d 3 -3))
  
  (defclass breaker (game)
    ((bricks :accessor bricks)
  