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;; Given

(define-struct add (mleft nright))
(define-struct sub (mleft nright))
(define-struct mul (mleft nright))
(define-struct div (mleft nright))

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