$ cat stuff-box.scm 
(module stuff-box ()

(import scheme)

(define (stuff-box)
  (let ((stuff '()))
    (lambda (cmd . args)
      (case cmd
        ((stuff) stuff)
        ((add-stuff!) (set! stuff (append args stuff)))))))

(define stuff (stuff-box))
(stuff 'add-stuff! "foo")
(car (stuff 'stuff))

)


$ csc stuff-box.scm 

Warning: Invalid argument
  In file `stuff-box.scm:14',
  At the toplevel,
  In procedure call:

    (scheme#car (stuff-box#stuff 'stuff))

  Argument #1 to procedure `car' has an invalid type:

    undefined

  The expected type is:

    pair

  It is a call to `stuff' from module `stuff-box' which has this type:

    (* #!rest -> undefined)

  This is the expression:

    (stuff-box#stuff 'stuff)

  Procedure `car' from module `scheme' has this type:

    ((pair 'a *) -> 'a)