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: 
    (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))
  