연습문제 2.66

(define (look-up given-key set-of-records)
  (cons ((null? set-of-records?) #f)
        (else
         (let ((cur-entry (entry set-of-records))
           (let ((cur-key (key cur-entry)))
             (cond ((= cur-key given-key) cur-entry)
                   ((< cur-key given-key)
                    (look-up given-key
                             (left-branch set-of-records)))
                   ((> cur-key given-key)
                    (look-up given-key
                             (right-branch set-of-records))))))))))