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; A natural (N) is either:
; - 0
; - (add1 n)
; where n is a natural

; Template
; nat-f : natural -> ...
;(define (f ... n ... ) 
;  (cond [(zero? n) ...] 
;        [(positive? n)
;         ... (f ... (sub1 n) ... ) ...]))

;; Given

(define-struct add (m n)) 
(define-struct sub (m n)) 
(define-struct mul (m n)) 
(define-struct div (m n)) 

;; an Arithmetic-Expression (AExp) is either:
;; - a number ;
;; - (make-add m n) where m,n are AExps;
;; - (make-sub m n where m,n are AExps; 
;; - (make-mul m n where m,n are AExps; or
;; - (make-div m n where m,n are AExps,