[plt-scheme] DrScheme Structures

Hello all,
When I define this:

;; distance-to-0 : posn  ->  number
;; to compute the distance of a-posn to the origin
(define (distance-to-0 a-posn) ...)


I get this error messege on clicking run;

...: name is not defined, not an argument, and not a primitive name.

Can someone please tell me what I am doing wrong?
Thanx.

-- 
Live, Learn, Love!
Patrick Amkayah.
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