anchor.diff:  (defun update (new-pos old-pos vel delta-time damping num-bodies p)
     (integrate-nbody-system new-pos old-pos vel delta-time damping num-bodies p))
   
  -#|
  -(defun run-benchmark (new-pos old-pos vel delta-time damping num-bodies p iterations)
  -  (update new-pos old-pos vel delta-time damping num-bodies p)
  -  ;; create-timer
  -  ;; start-timer
  -  (loop repeat iterations
  -     do (update new-pos old-pos vel delta-time damping new-pos p))
  -  ;; stop-timer
  -  ;; get elapsed time
  -  ;; delete-timer
  -  ;; compute-perf-stats
  -  ;; show-log
  -  )
  -|#
  -
   (defclass nbody-window (glut:window)
     ((new-pos :initform nil)
      (old-pos :initform nil)
  
positive.code:    (delta-time :initform 0)
     (damping :initform 0)
     (num-bodies :initform 0)
     (p :initform 0)
     (start-event :initform nil)
     (stop-event :initform nil))
    (:default-initargs :width 640 :height 480 :pos-x 100 :pos-y 100
                       :mode '(:double :rgb) :title "nbody"))
  
  
negative.code:       (let ((p (mem-aref old-pos i)))
          (gl:vertex (float4-x p) (float4-y p) (float4-z p)))))
    (gl:end)
    ;; swap buffers
    (glut:swap-buffers)
    ;; display frame rate
    (display-frame-rate (slot-value w 'num-bodies)))
  
  (defvar *flops-per-interaction* 20)
  
  