anchor.diff:  
     (update-program-matrices))
   
  -(defevent clear ()
  +(defun clear ()
     ;; reset turtle
     ;; reset line-drawer
     ;; reset camera
  
positive.code: (defun back (dist)
    (forward (- dist)))
  
  (defevent rotate-turtle (vec)
      (includef *turtle* :rotation (vec3f+ (@ *turtle* :rotation) vec)))
  
  (defun left (radians)
    ;; rotate turtle around z-axis
  
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))
  