(define (postfix-eval lst)
  (define (bereken l)
    (let ((stack (stack:new))
          (current (car l)))
      (cond ((number? current) (stack:push! stack current))
            ((null? l) (stack:top stack))
            (else (let ((getal1 (stack:pop! stack))
                        (getal2 (stack:pop! stack)))
                    (stack:push! (current getal1 getal2))
                    (bereken (cdr l)))))))
  (bereken lst))