[racket] use variable(in separated file) mutually ?

hello~
I was told by mailing list that read the racket guide unit section
but sorry I still don't understand.

the following code works very well
;;content of "a.rkt"
#lang racket
(provide a)
(define a 1)

;;content of "b.rkt"
#lang racket
(provide b)
(define b 2)

;;content of "ab.rkt"
#lang racket
(require "a.rkt" "b.rkt")
a
b

but I wonder how to use variable a in b.rkt and use b in a.rkt
with be able to use both a and b in ab.rkt

as like follow
;;content of "a.rkt"
#lang racket
(provide a)
(define a 1)
(display b)

;;content of "b.rkt"
#lang racket
(provide b)
(define b 2)
(display a)

;;content of "ab.rkt"
#lang racket
(require "a.rkt" "b.rkt")
(display a)
(display b)


could somebody tell me little bit more specify?
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.racket-lang.org/users/archive/attachments/20101113/b6d570f2/attachment.html>