anchor.diff:      (let ((pos (body-system-old-pos system))
             (num-bodies (body-system-num-bodies system)))
         (display-particle-renderer renderer pos num-bodies :particle-sprites))))
  -
  -(defun nbody-demo-measure-framerate (demo)
  -  (body-system-measure-framerate (nbody-demo-system demo)))
   
   (defun nbody-demo-num-bodies (demo)
     (body-system-num-bodies (nbody-demo-system demo)))
  
positive.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)))
  
  (defun body-system-measure-framerate (system)
    (measure-framerate-counter (body-system-framerate-counter system)))
  
negative.code:     ;; swap buffers
      (glut:swap-buffers)
      ;; display frame rate
      (display-framerate (nbody-demo-fps nbody-demo)
                         (nbody-demo-num-bodies nbody-demo))))
  
  