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)))
  +    (rotatef new-pos old-pos))
  +  ;; Measure framerate
  +  (body-system-measure-framerate))
   
   (defun body-system-measure-framerate (system)
     (measure-framerate-counter (body-system-framerate-counter system)))
  
positive.code:     (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)))
  
negative.code:     ;; swap buffers
      (glut:swap-buffers)
      ;; display frame rate
      (display-framerate (nbody-demo-fps nbody-demo)
                         (nbody-demo-num-bodies nbody-demo))))
  
  