[plt-scheme] Unable to expand to a correct prefix in macro...

Hi all -

at the risk of beating this point (
http://list.cs.brown.edu/pipermail/plt-scheme/2007-April/017471.html) to
death, I've found that a possible issue is that my macro doesn't seem to do
what I expected... namely add an automatic prefix to a required lib.  Any
pointers are greatly appreciated.

What I am trying to write...

(require-prefix (lib "test.ss" "test"))

Which should expand out to

(require (prefix test.test.ss: (lib "test.ss" "test")))

But it isn't working... I've simplify the test code to add a static prefix
(a:) rather than automatically generating a prefix but still have issues - I
thought with-syntax would ensure the prefix gets expanded with the correct
syntax-info, but there must be other points that I don't understand that
currently escaped me on why this macro doesn't work...

  (define-syntax require-prefix
    (lambda (stx)
      (syntax-case stx ()
        ((_ req-clause)
         (with-syntax ((pref (datum->syntax-object stx (string->symbol
"a:"))))
         #`(require (prefix pref req-clause))))
        ((_ req-clause rest ...)
         #'(begin
             (_ req-clause)
             (_ rest ...))))))

;; REPL
> (require-prefix (lib "string.scm" "util"))
> a:substr
Error. reference to undefined identifier: a:substr
> (require (prefix a: (lib "string.scm" "util")))
> a:substr
#<procedure:substr>

Thanks,
yinso
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