1 (define (make-table)
2   (define local-table '())
3   (define make-record cons)
4   (define key-record car)
5   (define value-record cdr)
6   (define (make-tree entry left right) 
7     (list entry let right)) 
8   (define entry car)
9   (define left-branch cadr)
10   (define right-branch caddr)
11   (define key=? equal?)
12   (define (list<? l1 l2)
13     (andmap key<? l1 l2))
14   (define (key<? key1 key2)
15     (cond ((and (string? key1)
16 		(string? key2)) (string<? key1 key2))
17 	  ((and (number? key1)
18 		(number? key2)) (< key1 key2))
19 	  ((and (char? key1)
20 		(char? key2)) (char<? key1 key2))
21 	  (else (error "Unsupported key types -- KEY<?" key1 key2))))
22   (define (element-of-set? x set)
23     (cond ((null? set) false)
24 	  ((key=? (key-record x) (key-record (entry set))) true)
25 	  ((key<? (key-record x) (key-record (entry set))) 
26 	   (element-of-set? x (left-branch set)))
27 	  (else
28 	   (element-of-set? x (right-branch set)))))
29   (define (adjoin-set x set)
30     (cond ((null? set) (make-tree x '() '()))
31 	  ((key=? (key-record x) (key-record (entry set))) set)
32 	  ((key<? (key-record x) (key-record (entry set)))
33 	   (make-tree (entry set)
34 		      (adjoin-set x (left-branch set))
35 		      (right-branch set)))
36 	  (else
37 	   (make-tree (entry set)
38 		      (left-branch set)
39 		      (adjoin-set x (right-branch set))))))
40   (define (lookup key records)
41     (if (null? records)
42 	false
43 	(let* ((record (entry records))
44 	       (key-entry (key-record record)))
45 	  (cond ((key=? key key-entry) (value-record record))
46 		((key<? key key-entry) (lookup key (left-branch records)))
47 		(else (lookup key (right-branch records)))))))
48   (define (insert! key value)
49     (set! local-table
50 	  (adjoin-set (cons key value)
51 		      local-table)))
52   (define (dispatch m)
53     (cond ((eq? m 'lookup)
54 	   (lambda (key)
55 	     (lookup key local-table)))
56 	  ((eq? m 'insert!) insert!)
57 	  ((eq? m 'print) local-table)
58 	  (else (error "Unknown operation -- TABLE" m))))
59   dispatch)
60