1 (define (entry tree) (car tree))
2 (define (left-branch tree) (cadr tree))
3 (define (right-branch tree) (caddr tree))
4 (define (make-tree entry left right)
5   (list entry left right))
6 
7 (define (element-of-set? x set)
8   (cond ((null? set) #f)
9 	((= x (entry set)) #t)
10 	((< x (entry set))
11 	 (element-of-set? x (left-branch set)))
12 	((> x (entry set))
13 	 (element-of-set? x (right-branch set)))))
14 
15 (define (adjoin-set x set)
16   (cond ((null? set) (make-tree x '() '()))
17 	((= x (entry set)) set)
18 	((< x (entry set))
19 	 (make-tree (entry set)
20 		    (adjoin-set x (left-branch set))
21 		    (right-branch set)))
22 	((> x (entry set))
23 	 (make-tree (entry set)
24 		    (left-branch set)
25 		    (adjoin-set x (right-branch set))))))
26 
27 ;; Exercise 2.66.  Implement the lookup procedure for the case where the set of records is structured as a binary tree, ordered by the numerical values of the keys. 
28 
29 (define (lookup given-key set-of-records)
30   (if (null? set-of-records)
31       #f
32       (let ((record (entry set-of-records))
33 	    (record-key (key record)))
34 	(cond ((= given-key record-key) 
35 	       record)
36 	      ((< given-key record-key)
37 	       (lookup given-key (left-branch set-of-records)))
38 	      ((> given-key record-key) 
39 	       (lookup given-key (right-branch set-of-records)))))))