anchor.diff:            (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)))
  +
  +(defun nbody-demo-fps (demo)
  +  (body-system-fps (nbody-demo-system demo)))
   
   
   ;;;
  
positive.code:   (symbol-macrolet ((new-pos (body-system-new-pos system))
                      (old-pos (body-system-old-pos system)))
      (rotatef new-pos old-pos)))
  
  
  ;;;
  
negative.code:         (cluster-scale (body-system-cluster-scale system))
          (velocity-scale (body-system-velocity-scale system))
          (num-bodies (body-system-num-bodies system)))
      (randomize-bodies old-pos vel
                        cluster-scale velocity-scale num-bodies)))
  
  (defun sync-body-system (system direction)
    (sync-array (body-system-new-pos system) direction)
  