;; IMPLEMENTIERUNG DER STROEMEE FUER SERIE 11, AUFGABE 42:

(define der-leere-strom '())
(define cons-strom cons)
(define leerer-strom? null?)
(define kopf car)
(define rest cdr)

(define (append-stroeme strom1 strom2)
  (if (leerer-strom? strom1)
      strom2
      (cons-strom (kopf strom1) (append-stroeme (rest strom1) strom2))))

(define (abb fun strom)
  (if (leerer-strom? strom)
      der-leere-strom
      (cons-strom (fun (kopf strom)) (abb fun (rest strom)))))

(define (filter pred strom)
  (cond ((leerer-strom? strom) der-leere-strom)
        ((pred (kopf strom))
         (cons-strom (kopf strom) (filter pred (rest strom))))
        (else (filter pred (rest strom)))))

(define (akk op init strom)
  (if (leerer-strom? strom)
      init
      (op (kopf strom) (akk op init (rest strom)))))