anchor.diff:  ;;;
   
   (defclass nbody-window (glut:window)
  -  ((nbody-demo :initarg :nbody-demo))
  +  ((nbody-demo :initarg :nbody-demo)
  +   (counter :initarg :counter))
     (:default-initargs :width 640 :height 480 :pos-x 100 :pos-y 100
                        :mode '(:double :rgb) :title "nbody"))
   
  
positive.code: |#
  
  (defun main (&key (gpu t) (interop nil))
    (let ((glut:*run-main-loop-after-display* nil)
          (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)
        (setf (slot-value window 'nbody-demo) demo)
        (glut:main-loop))))
  
negative.code: ;  nil)
  
  (defmethod glut:display ((w nbody-window))
    (with-slots (nbody-demo counter) w
      ;; update simulation
      (update-nbody-demo nbody-demo)
      ;; clear buffers
  