anchor.diff:  (defun game-step ()
     (iterate #'(lambda (x y)
   	       (let ((count (game-count-alive x y))
  -		     (alive (matrix-get *matrix* x y)))
  +		     (alive (matrix-get *old-matrix* x y)))
   		 (if alive
   		     (if (or (= count 2) (= count 3))
   			 'survives
  
positive.code:     (:key-down-event () (sdl:push-quit-event))
      (:idle
       ()
       (game-step)
       (draw-matrix)
       (sdl:update-display)))))
  
negative.code: (defun game-count-alive (x0 y0)
    (let ((alive-count 0))
      (iterate-near #'(lambda (x y)
  		      (when (matrix-get *old-matrix* x y)
  			(setf alive-count (add1 alive-count))))
  		  x0
  		  y0)
      alive-count))
  
  (defun update-old-matrix ()
    (iterate #'(lambda (x y)
  	       (matrix-set *old-matrix*
  			   x y
  			   (matrix-get *matrix* x y)))
  	   *cells-count*))
  
  ;;; Draw
  ;;; ====
  