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;; Given

(define-struct add (left right))
(define-struct sub (left right))
(define-struct mul (left right))
(define-struct div (left right))

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

;; Remember that the define-struct function also automatically defines comparison functions:
;; add?, sub?, mul? and div?

;; Note: the function, number?, can be used to test if a value is a number.