; FamilyTree -> Names
; extract the names of all mothers and fathers from a family tree
(define (names-of-ancestors family-tree)
  (match family-tree
    [(person name mother father)
     (local [(define (append-parent parent selector)
               (local [(define (select-parents parent) (selector (names-of-ancestors parent)))]
                 (make-names (append (if (person? parent)
                                         (list (person-name parent))
                                         (list))
                                     (get-parents mother))
                             (get-parents father))))]
       (make-names (append-parent mother names-mothers)
                   (append-parent father names-fathers)))]
    [#false
     (make-names (list) (list))]))