[plt-scheme] transparent background in an editor-canvas%

we are on version 203 under win 2k  and we'd like to
have a transparent background in an editor-canvas% we
have wrote : 

(define ec (instantiate editor-canvas% () (parent
frame)))
(define dc (send ec get-dc))
(define txt (instantiate text%))
(send dc set-background (make-object color%
"transparent"))
(send dc set-text-foreground (make-object color%
"BLUE"))
(send ec set-editor txt)

and the text is black on a white background.

can anyone tell us where is the mistake ?

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