(datatype arith-expr

    X : number >> Y : A;
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    [num X] : arith-expr >> Y : A;

    X : number;
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    [num X] : arith-expr; 

    if (element? Op [+ - * /])
    X : arith-expr; Y : arith-expr;
    ======================
   	[X Op Y] : arith-expr;)
   	    
(define do-calculation
   {arith-expr --> number}
    [X + Y] -> (+ (do-calculation X) (do-calculation Y))
    [X - Y] -> (- (do-calculation X) (do-calculation Y))
    [X * Y] -> (* (do-calculation X) (do-calculation Y))
    [X / Y] -> (/ (do-calculation X) (do-calculation Y))
    [num X] -> X)