SICP 問題 2.66
(define (lookup-tree given-key set-of-records) (cond (let ((key-record (key (car set-of-records)))) ((null? set-of-records) false) ((= given-key key-record) (car set-of-records)) ((< given-key key-record) (lookup-tree given-key (left-branch set-of-records))) ((< key-record given-key) (lookup-tree given-key (right-branch set-of-records))))))