#| BEGIN (Введите свое решение) |#
(define (lookup given-key tree)
  (cond [(null? tree) #f]
        [(< given-key (key (entry tree))) (lookup given-key (left-branch tree))]
        [(> given-key (key (entry tree))) (lookup given-key (right-branch tree))]
        [(equal? given-key (key (entry tree))) (entry tree)]))


#| END |#