anchor.diff:  (defun collada-povray (&key
                            (dom *collada-dom*)
                            file)
  -  (let ((result (pov-sequence
  -                 (let* ((node (collada-node-singleton dom "library_visual_scenes")))
  +  (let* ((result (pov-sequence
  +                 (let* ((node (dom-select-tag dom "library_visual_scenes" :singleton t)))
                      (collada-povray-visual-scene dom node)))))
       (if file
           (with-open-file (s file :direction :output :if-exists :supersede)
  
positive.code: 
  (defun collada-parse-material (dom node)
    (assert (string= (dom:tag-name node) "material"))
    (let ((instance-effect (collada-node-singleton node "instance_effect")))
      (collada-attr-id-parse dom instance-effect "url")))
  
  
  
negative.code: 
  (defun collada-parse-vertices (dom node)
    (assert (string= (dom:tag-name node) "vertices"))
    (let ((input (dom-select-tag node "input" :direct t :singleton t)))
      (assert (and (dom:has-attribute input "semantic")
                   (string= (dom:get-attribute input "semantic")
                            "POSITION")))
  