;; update-year : ftnode string number -> ftnode
  ;; update year of named person in given tree (assume person in tree
  ;; at most once)

;; Test cases
(check-expect (update-year 'unknown "Mary" 1981) unknown)

(check-expect (update-year MaryTree "Mary" 1981)
             (make-person "Mary" 1981 'blue
                          (make-person "Ann" 1960 'green
                                       'unknown
                                       (make-person "Fred" 1936 'blue
                                                    'unknown
                                                    'unknown))
                          (make-person "Joe" 1960 'blue
                                       'unknown
                                       'unknown)))


;; update-year : ftnode string number -> ftnode
;; update year of named person in given tree (assume named person in tree
;; at most once)
(define (update-year aft name birthyear)
  (cond [(symbol? aft) 'unknown]
        [(person? aft)
         (cond [(string=? (person-name aft) name)
                (make-person name 
                             birthyear
                             (person-eye aft)
                             (person-mother aft)
                             (person-father aft))]
               [else (make-person (person-name aft) 
                                  (person-year aft) 
                                  (person-eye aft)
                                  (update-year (person-mother aft) name birthyear)
                                  (update-year (person-father aft) name birthyear))])]))