[plt-scheme] Modifying imports

$ cat foo.ss
(module foo mzscheme
   (define some-var 0)
   (provide some-var))
$ cat bar.ss
(require (prefix foo: "foo.ss"))

(set! foo:some-var 1)
$ mzscheme -r bar.ss
bar.ss:3:6: set!: cannot mutate module-required variable at: some-var 
in: foo:some-var

Is there any way to modify ``module-required'' variables ?

~velco