[plt-scheme] ...does not implement #<struct:|interface:host<%>|>

Hello

The program below (which is part of a larger program) works fine on
my Macintosh with DrScheme, Version 204. However, when I try to execute
it with DrScheme, Version 204.8-cvs17jul2003 german, I obtain the
following error message:

--------------------------------
text-mixin: argument #<struct:class:transcript:return%> does not implement #<struct:|interface:host<%>|>
--------------------------------

What is the problem? Here is the program:

--------------------------------
(module main mzscheme
  (require (lib "framework.ss" "framework"))
  (require (lib "mred.ss" "mred") (lib "class.ss"))

(define transcript:return%
  (class text:return% (init (return driver-loop))
    (super-instantiate () (return return))))

(define transcript:editor% (scheme:text-mixin transcript:return%))

(define (driver-loop)
  #t)

)
--------------------------------

Thanks very much
Norbert

-- 
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                                (o o)
+---------------------------oOO--(_)--OOo---------------------------+
Norbert Lehmann
Department of Informatics (DIUF)            phone: ++ 41 26 300 83 30
University of Fribourg                        fax: ++ 41 26 300 97 26
Rue Faucigny 2                       e-mail: Norbert.Lehmann at unifr.ch
CH-1700 Fribourg                    http://www-iiuf.unifr.ch/~lehmann
Switzerland
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