(define (reader)
 (case (read-a-thing)
   ((vertex) (add-vertex the-thing))
    ...))

(define (writer scene . things)
  (for-each
   (cut thing-writer scene <>)
   things))

(define (thing-writer scene thing)
  (case thing
     ((vertex) (write (accessor vertex scene)))
     ...))



(reader)

(writer scene vertex normals indexes)