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