anchor.diff:  
   (defvar *flops-per-interaction* 20)
   
  -(defun compute-pref-stats (counter num-bodies interactions)
  +(defun compute-pref-stats (fps num-bodies interactions)
     (let* ((interactions-per-second
              (/ (* num-bodies num-bodies interactions)
                 (/ 1.0 fps)
  
positive.code:   (new-pos :new-pos)                    ; Not read-only to flip
    (old-pos :old-pos)
    (vel :vel :read-only t)
    ;; Flags
    (gpu :gpu :read-only t)
    (interop :interop :read-only t)
  
negative.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"))
  
  