prefix->list

(define (prefix->list los)
  (local
    (
     ;list-tail: (listof symbol/number) number -> (listof symbol(number)
     ;Gib die Liste ab gegebenen Index wieder
     (define (list-tail loa num)
       (local
         (
          (define le (length loa))
          ;list-tail: (listof symbol/number) number  (listof symbol/number) -> (listof symbol/number)
          ;Gib die Liste ab gegebenen Index wieder
          (define (list-tail loa num akk)
            (cond
              [(> num (sub1 le)) (error 'list-tail "index reaches a non-pair index")]
              [(= num (sub1 le)) (if (empty? akk)
                                     (list(list-ref loa num))
                                     (append akk (list (list-ref loa num)) empty))]
              [else (list-tail loa (add1 num) (append akk (list(list-ref loa num))))]
              )
            )
          )
         (list-tail loa num empty)
         )
       )
     ;recursive-length: (listof number/symbol/list) -> number
     ;Gib die rekursive Länge einer Liste an
     (define (recursive-length ls)
       (if (empty? ls)
           0
           (if (not (list? (first ls)))
               (+ 1 (recursive-length (rest ls)))
               (+ (recursive-length (first ls)) (recursive-length (rest ls))))))
     (define operator (or (eq? (first los) '+)
                          (eq? (first los) '-)
                          (eq? (first los) '*)
                          (eq? (first los) '/)))
     ;dice: symbol -> boolean
     ;Abfrage, ob es ein Würfel ist
     (define (dice d) (or (eq? d 'w4)
                          (eq? d 'w6)
                          (eq? d 'w8)
                          (eq? d 'w10)
                          (eq? d 'w12)
                          (eq? d 'w20)
                          (eq? d 'w100)))
     ;attribute: symbol -> boolean
     ;Abfrage, ob es sich um ein Attribut handelt
     (define (attribute a) (or (eq? a '%CO)
                               (eq? a '%RE)
                               (eq? a '%ST)
                               (eq? a '%IN)
                               (eq? a '%CH)))
     )
    (cond
      [(or (number? (first los)) (dice (first los)) (attribute (first los))) (first los)]
      [operator
       (local
         (
          (define first-operator (prefix->list (rest los)))
          (define second-operator
            (if (or (number? first-operator)(dice first-operator)(attribute first-operator))
                (prefix->list (list-tail los 2))
                (prefix->list (list-tail los (+ 1 (recursive-length first-operator))))))
          )
         (list
          (first los)
          first-operator
          second-operator))]
      )
    )
  )