anchor.diff:  
   (defclass paddle (color-rect) ())
   
  -(defclass ball (obj-circle has-color has-velocity) ())
  +(defclass ball (obj-circle has-color has-velocity)
  +  ((attached :initform T :initarg :attached :accessor attached)))
   
   (defmethod add ((obj1 vector-2d) (obj2 vector-2d))
     (vector-2d (+ (x obj1) (x obj2)) (+ (y obj1) (y obj2))))
  
positive.code:       (sdl:rectangle-from-edges-* (x loc) (y loc) (x corner) (y corner))
        :color (color obj))))
  
  ;; parameters
  (defparameter *paddle-speed* 8)
  
  (defclass breaker (game)
    ((bricks :accessor bricks)
  
negative.code:   (setf (balls game) nil))
  
  (defmethod step-game ((game breaker) keys-down)
    ;; update paddle
    (with-accessors ((x-paddle x)) (location (paddle game))
      (dolist (key keys-down)
        (case key
          (:left (decf x-paddle *paddle-speed*))
          (:right (incf x-paddle *paddle-speed*))
          (:up (when (count-if #'attached (balls game))
                 (mapc (lambda (b)
                         (when (attached b)
                           (setf (attached b) nil)  
                           (setf (velocity b)
                                 *ball-init-velocity*)))
                       (balls game))))
          ))
      (setf x-paddle (max x-paddle 0))
      (setf x-paddle (min x-paddle (- (x (field-dim game))
                                      (x (dimensions (paddle game)))))))
    ;; update balls
    (when (null (balls game))
      (push (make-instance 'ball :radius *ball-radius*) (balls game)))
    (setf (balls game)
          (mapcar (lambda (b)
                    (if (attached b)
                      (progn
                        (setf (location b)
                              (add (location (paddle game))
                                  (vector-2d
                                    (floor (x (dimensions (paddle game))) 2)
                                    (* -1 (radius b)))))
                        b)
                      (progn
                        (setf (location b)
                              (add (location b)
                                   (velocity b)))
                        b)
                      ))
                  (balls game)))
    )
  
  (defmethod draw-game ((game breaker))
    (dolist (brick (bricks game))
  