anchor.diff:          *camera* (make-init-camera)
           *line-drawer* (make-line-drawer (get-program "line"))))
   
  -(defevent pen-toggle ()
  +(defun pen-toggle ()
     (includef *turtle* :pen-down-p (not (@ *turtle* :pen-down-p))))
   
   (defun pen-down ()
  
positive.code: 
    (update-program-matrices))
  
  (defevent clear ()
    ;; reset turtle
    ;; reset line-drawer
    ;; reset camera
  
negative.code:   (when (@ *turtle* :pen-down-p)
      (pen-toggle)))
  
  (defun color (vec)
    (includef *turtle* :color (vec4f vec)))
  
  (defun forward (distance)
    (let ((new-pos (vec3f (kit.glm:matrix*vec4
                           (kit.glm:matrix*
                            (kit.glm:translate (@ *turtle* :position))
  