(define (fringe tree)
  (cond ((null? tree) '())
        ((atom? tree) (list tree))
        (else (append (fringe (car tree))
                      (fringe (cdr tree))))))
;; 7.11.3
(define (collegas p organigram)
(define (collegas organigram alle-collegas)
    (if (eq? (baas organigram) p)
        (fringe (append alle-collegas (sub-organ organigram)))
        (collegas-in (sub-organ organigram) (cons (baas organigram) alle-collegas))))
  (define (collegas-in organigram alle-collegas)
    (if (null? organigram)
        #f
        (or (collegas (car organigram) alle-collegas)
            (collegas-in (cdr organigram) alle-collegas))))
  (collegas organigram '()))

;; mogen wij deze oefening met behulp van fringe op deze manier oplossen?