問題2.66
(define (lookup given-key set-of-records) (if (null? set-of-records) #f (let ((record (entry set-of-records))) (cond ((= given-key (key record)) record) ((< given-key (key record)) (lookup given-key (left-branch set-of-records))) ((> given-key (key record)) (lookup given-key (right-branch set-of-records)))))))