(define eval-arith
  '(let (member?
         (lambda-rec (member-in-list? e list)
                     (cond
                       [(eq? list `()) false]
                       [(eq? e (car list)) true]
                       [true (member-in-list? e (cdr list))])))

     (let
         (length
          (lambda-rec (lenght list)
                      (if (eq? list `())
                          0
                          (+ 1 (len (cdr list))))))
       (let
           (is-list?
            (lambda-rec (is-list? list)
                        (or (eq? list `())
                            (and (pair? list)
                                 (is-list? (cdr list))))))

         (let
             (binop?
              (lambda (e)
                (and (is-list? e)
                     (and 😊 (length e) 3)
                          (member? (car e) (cons-cons '+ (cons-cons '- (cons-cons '* (cons-cons '/ null)))))))))