[plt-scheme] basic syntax problem

hi, I'm having unexpected syntax trouble in something that worked before, so i suspect i am breaking rules i'm not aware of.

the following code:
  (require  ( lib "url.ss" "net")) 

  (define (f u)
    (string->url u))

compiles once (and then runs), but not twice. the second attempt to "start"
 yields (cf. below):
   compile: bad syntax; reference to top-level identifier is not allowed, because no #%top syntax transformer is bound in: lifted

could someone please explain or point me to a good doc? (i have dug a bit, but couldn't find anything.)

cheers,

Tim

Willkommen bei DrScheme, Version 372 [3m].
Sprache: Kombo (enthält MrEd and Fortgeschritten) angepasst.
> (f "www.faz.net")
#<struct:url>
----------------------------
Willkommen bei DrScheme, Version 372 [3m].
Sprache: Kombo (enthält MrEd and Fortgeschritten) angepasst.
compile: bad syntax; reference to top-level identifier is not allowed, because no #%top syntax transformer is bound in: lifted
> 
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