anchor.diff:  (defun fold-scene-graph-frames (function initial-value scene-graph)
     (fold-tree-set function initial-value (scene-graph-frames scene-graph)))
   
  -(defun prefix-scene-graph (prefix scene-graph)
  +(defun prefix-scene-graph (prefix scene-graph &key
  +                                                (prefix-parents t))
     (let ((frames))
  -    (do-scene-graph-frames (frame scene-graph (scene-graph frames))
  +    (do-scene-graph-frames (frame (scene-graph scene-graph) (scene-graph frames))
         (let ((frame (copy-structure frame))
               (parent (scene-frame-parent frame)))
           (setf (scene-frame-name frame)
                 (rope prefix (scene-frame-name frame)))
  -        (when parent
  +        (when (and parent prefix-parents)
             (setf (scene-frame-parent frame)
                   (rope prefix parent)))
           (when (scene-frame-joint-p frame)
  
positive.code:     (:color . (0 0 0))
      (:alpha . 1d0)
      (:visual . t)
      (:collision . t)))
  
  (defun draw-option (options key)
    (alist-get-default options key *draw-options*))
  
negative.code: (defun scene-geometry (shape options)
    (make-scene-geometry :shape shape
                         :options options
                         :type (let ((type (draw-option options :type)))
                                 (etypecase type
                                   (list (apply #'tree-set #'rope-compare-fast type))
                                   (tree-set type)
                                   (rope (tree-set #'rope-compare-fast type))))
                         :collision (draw-option options :collision)
                         :visual (draw-option options :visual)))
  
  