;;evaluate
(define (evaluate exp)
  (cond ((number? exp) exp)
        (else (if (or (null? exp) (and (symbol? exp) (not (or (equal? exp `+) (equal? exp `-) (equal? exp `*) (equal? exp `/))))) "undefined"
                  (if (list? (car exp)) (evaluate (multi-replace (car exp) (car (cdr exp))))
                      (if (equal? (car exp) `+) (add (evaluate (car (cdr exp))) (evaluate (car (cdr (cdr exp)))))
                          (if (equal? (car exp) `-) (minus (evaluate (car (cdr exp))) (evaluate (car (cdr (cdr exp)))))
                              (if (equal? (car exp) `*) (multiply (evaluate (car (cdr exp))) (evaluate (car (cdr (cdr exp)))))
                                  (if (equal? (car exp) `/) (divide (evaluate (car (cdr exp))) (evaluate (car (cdr (cdr exp)))))
                                      "undefined")))))))))

(define (add x y)
  (cond ((or (equal? "undefined" x) (equal? "undefined" y)) "undefined")
        (else (+ x y))))
(define (minus x y)
  (cond ((or (equal? "undefined" x) (equal? "undefined" y)) "undefined")
        (else (- x y))))
(define (divide x y)
  (cond ((or (equal? "undefined" x) (equal? "undefined" y)) "undefined")
        (else (/ x y))))
(define (multiply x y)
  (cond ((or (equal? "undefined" x) (equal? "undefined" y)) "undefined")
        (else (* x y))))

(define (multi-replace x y)
  (cond ((null? x) y)
        (else (multi-replace (cdr x) (replace-helper (car x) y)))))

(define (replace-helper e exp)
  (list-replace exp (car e) (car (cdr e))))