anchor.diff:  ;;;
   
   (defclass nbody-window (glut:window)
  -  ((nbody-demo :initarg :nbody-demo))
  +  ((nbody-demo :initarg :nbody-demo)
  +   (counter :initarg :counter))
     (:default-initargs :width 640 :height 480 :pos-x 100 :pos-y 100
                        :mode '(:double :rgb) :title "nbody"))
   
  
positive.code:     (display-nbody-demo nbody-demo)
      ;; swap buffers
      (glut:swap-buffers)
      ;; display frame rate
      (display-framerate (nbody-demo-fps nbody-demo)
                         (nbody-demo-num-bodies nbody-demo))))
  
  (defmethod glut:reshape ((w nbody-window) width height)
    ;; configure on projection mode
  
negative.code:   ;; Flip position arrays
    (symbol-macrolet ((new-pos (body-system-new-pos system))
                      (old-pos (body-system-old-pos system)))
      (rotatef new-pos old-pos)))
  
  
  ;;;
  