anchor.diff:  (defun back (dist)
     (forward (- dist)))
   
  -(defevent rotate-turtle (vec)
  -    (includef *turtle* :rotation (vec3f+ (@ *turtle* :rotation) vec)))
  +(defun rotate-turtle (vec)
  +  (includef *turtle* :rotation (vec3f+ (@ *turtle* :rotation) vec)))
   
   (defun left (radians)
     ;; rotate turtle around z-axis
  
positive.code:         *camera* (make-init-camera)
          *line-drawer* (make-line-drawer (get-program "line"))))
  
  (defevent pen-toggle ()
    (includef *turtle* :pen-down-p (not (@ *turtle* :pen-down-p))))
  
  (defun pen-down ()
  
negative.code: 
    (update-program-matrices))
  
  (defun clear ()
    ;; reset turtle
    ;; reset line-drawer
    ;; reset camera
  