(define (safe-eval ex)
  (cond
    [(number? ex) ex]
    [(string? ex) ex]
    [(bae? ex)
     (cond
       [(or(string? (bae-arg1 ex))(string? (bae-arg2 ex))) "divide by zero"]
       [else
     (cond
       [(symbol=? (bae-fn ex) '*)
               (cond [(or(string? (safe-eval (bae-arg1 ex)))(string?(safe-eval(bae-arg2 ex)))) "divide by zero"]
              [else (*(safe-eval (bae-arg1 ex))(safe-eval(bae-arg2 ex)))])]

       [(symbol=? (bae-fn ex) '+)
              (cond [(or(string? (safe-eval (bae-arg1 ex)))(string?(safe-eval(bae-arg2 ex)))) "divide by zero"]
              [else (+(safe-eval (bae-arg1 ex))(safe-eval(bae-arg2 ex)))])]

       [(symbol=? (bae-fn ex) '/)
        (cond [(equal? (safe-eval(bae-arg2 ex)) 0) "divide by zero"]
              [(or(string? (bae-arg1 ex))(string? (bae-arg2 ex))) "divide by zero"]
              [else (/(safe-eval (bae-arg1 ex))(safe-eval(bae-arg2 ex)))])]

       [(symbol=? (bae-fn ex) '-)
             (cond [(or(string? (safe-eval (bae-arg1 ex)))(string?(safe-eval(bae-arg2 ex)))) "divide by zero"]
              [else (-(safe-eval (bae-arg1 ex))(safe-eval(bae-arg2 ex)))])])])]))

(check-expect (safe-eval (make-bae '* 2 6))12)
(check-expect (safe-eval (make-bae '/ 2 0))"divide by zero")
(check-expect (safe-eval (make-bae '+ 2 (make-bae '/ 5 0)))"divide by zero")
(check-expect (safe-eval (make-bae '/ (make-bae '/ (make-bae '* 5 0)(make-bae '* 5 2))(make-bae '- 5 2)))0)
(check-expect (safe-eval (make-bae '/ 1 (make-bae '- 5 5)))"divide by zero")