1 (define (test-case actual expected)
2   (newline)
3   (display "Actual:   ")
4   (display actual)
5   (newline)
6   (display "Expected: ")
7   (display expected)
8   (newline))
9 
10 (define (scale-tree tree factor)
11   (cond ((null? tree) '())
12 	((not (pair? tree)) (* factor tree))
13 	(else (cons (scale-tree (car tree) factor)
14 		    (scale-tree (cdr tree) factor)))))
15 
16 (define (scale-tree tree factor)
17   (map (lambda (sub-tree)
18 	 (if (pair? sub-tree)
19 	     (scale-tree sub-tree factor)
20 	     (* factor sub-tree)))
21        tree))
22 
23 (define (square-tree tree)
24   (cond ((null? tree) '())
25 	((not (pair? tree)) (* tree tree))
26 	(else (cons (square-tree (car tree))
27 		    (square-tree (cdr tree))))))
28 
29 ;; (test-case (square-tree
30 ;; 	    (list 1
31 ;; 		  (list 2 (list 3 4) 5)
32 ;; 		  (list 6 7)))
33 ;; 	   '(1 (4 (9 16) 25) (36 49)))
34 
35 ;; (define (square-tree-map tree)
36 ;;   (map (lambda (sub-tree)
37 ;; 	 (if (pair? sub-tree)
38 ;; 	     (square-tree-map sub-tree)
39 ;; 	     (* sub-tree sub-tree)))
40 ;;        tree))
41 
42 ;; (test-case (square-tree-map
43 ;; 	    (list 1
44 ;; 		  (list 2 (list 3 4) 5)
45 ;; 		  (list 6 7)))
46 ;; 	   '(1 (4 (9 16) 25) (36 49)))
47 
48 ;; Exercise 2.31.  Abstract your answer to exercise 2.30 to produce a procedure tree-map with the property that square-tree could be defined as
49 
50 (define (tree-map proc tree)
51   (cond ((null? tree) '())
52 	((not (pair? tree)) (proc tree))
53 	(else (cons (tree-map proc (car tree))
54 		    (tree-map proc (cdr tree))))))
55 
56 (define (square-tree-map tree) (tree-map square tree))
57 
58 (test-case (square-tree-map
59 	    (list 1
60 		  (list 2 (list 3 4) 5)
61 		  (list 6 7)))
62 	   '(1 (4 (9 16) 25) (36 49)))
63 
64 
65 ;; Exercise 2.32.  We can represent a set as a list of distinct elements, and we can represent the set of all subsets of the set as a list of lists. For example, if the set is (1 2 3), then the set of all subsets is (() (3) (2) (2 3) (1) (1 3) (1 2) (1 2 3)). Complete the following definition of a procedure that generates the set of subsets of a set and give a clear explanation of why it works:
66 
67 (define (subsets s)
68   (if (null? s)
69       (list nil)
70       (let ((rest (subsets (cdr s))))
71 	(append rest (map <??> rest)))))