anchor.diff:             (type single-float delta-time damping)
              (type vec3-array new-pos old-pos vel))
     (dotimes (i total-num-bodies)
  -    (with-vec3-array-values ((x1 y1 z1) (old-pos i))
  +    (with-vec3-array-values ((x1 y1 z1) old-pos i)
         (dotimes (j total-num-bodies)
  -        (with-vec3-array-values ((x2 y2 z2) (old-pos j))
  +        (with-vec3-array-values ((x2 y2 z2) old-pos j)
             (when (/= i j)
               (multiple-value-bind (ax ay az) (body-body-interaction-cpu x1 y1 z1 x2 y2 z2)
                 (declare (type single-float ax ay az))
                 (let ((k (* delta-time damping)))
  -                (with-vec3-array-values ((vx vy vz) (vel i))
  +                (with-vec3-array-values ((vx vy vz) vel i)
                     (set-vec3-array (vel i) (+ vx (* ax k))
                                             (+ vy (* ay k))
                                             (+ vz (* az k)))))))))
  -      (with-vec3-array-values ((vx vy vz) (vel i))
  +      (with-vec3-array-values ((vx vy vz) vel i)
           (set-vec3-array (new-pos i) (+ x1 (* vx delta-time))
                                       (+ y1 (* vy delta-time))
                                       (+ z1 (* vz delta-time)))))))
  
positive.code:         (window (make-instance 'nbody-window)))
      (glut:display-window window) ; GLUT window must be created before initializing nbody-demo
      (with-nbody-demo (demo 2048 :gpu gpu :interop interop)
        (with-frame-rate-counter (counter)
          (setf (slot-value window 'nbody-demo) demo
                (slot-value window 'counter)    counter)
          (glut:main-loop)))))
  
  
  (defun not-implemented ()
    (error "not implemented."))
  
negative.code:     ;; swap buffers
      (glut:swap-buffers)
      ;; display frame rate
      (measure-framerate counter)
      (display-framerate (nbody-demo-num-bodies nbody-demo)
                         (get-framerate counter))))
  
  (defmethod glut:reshape ((w nbody-window) width height)
    ;; configure on projection mode
  