can't export conditional variable set with 'cond'

[Mortimer Cladwell <mbcladwell@gmail.com>]

Hi,

Suppose I have the following module "testexport" with the auxiliary module
"env.scm", used to set global environment variables:

-------------testexport.scm begin----------------------------------
(define-module (testexport)
  #:use-module (ice-9 pretty-print)
  #:use-module (env)
  )

(define (main)
  (pretty-print (string-append  "varA in main: " varA))
  (pretty-print (string-append  "varB in main: " varB))
  (pretty-print (string-append  "varC in main: " varC))
  )

(main)
-------------testexport.scm end----------------------------------

-------------env.scm begin----------------------------------
(define-module (env)
  #:use-module (ice-9 pretty-print)
  #:export(varA varB varC))

(define varA "A")
(define varB "")
(define varC "")
(define testval "x")

(cond
 ((string= testval "x") (set! varB "B"))
 ((string= testval "y") (set! varB "error"))
 ((string= testval "z") (set! varB "null"))
 )
(pretty-print (string-append "varB post cond: " varB))

(if (string= testval "x")  (set! varC "C"))
(pretty-print (string-append "varC post if: " varC))
-------------env.scm end----------------------------------

When I run the above I get as output:

"varB post cond: B"
"varC post if: C"
"varA in main: A"
"varB in main: "
"varC in main: C"

Why can I reset the variables with both 'cond' and 'if' in env.scm, but
only the variable reset with 'if' is exported with the updated value?
Thanks
Mortimer