(define (explode-string str)
       (local
         (
          (define (operator o) (or (string=? o "+")
                                   (string=? o "-")
                                   (string=? o "*")
                                   (string=? o "/")))
          ;explode-string: string string (listof strings)-> (listof strings)
          ;Übersetzt einen string in einzelne Symbole und zusammenhängende Zahlen
          (define (explode-string2 str akk akk2)
            (cond
              [(empty? str) (append akk2 (list akk ))]
              [(operator (first str)) (if (string=? akk "")
                                          (explode-string2 (rest str) akk (append akk2 (list (first str))))
                                          (explode-string2 (rest str) "" (append akk2 (list akk)(list(first str)))))]
              [else (explode-string2 (rest str) (string-append akk (first str)) akk2)]
              )
            )
          )
         (explode-string2 (explode str) "" empty)
         )
       )