1 (defun make-sum (a1 a2)
2   (list a1 '+ a2))
3 (defun make-product (m1 m2)
4   (list m1 '* m2))
5 (defun sum? (x)
6   (and (consp x) (eql (cadr x) '+)))
7 (defun addend (x)
8   (car x))
9 (defun augend (s)
10   (caddr s))
11 (defun product? (x)
12   (and (consp x) (eql (cadr x) '*)))
13 (defun multiplier (s)
14   (car s))
15 (defun multiplicand (s)
16   (caddr s))
17 
18 (defvar *stream* '() "Token stream")
19 (defun init-stream (stream)
20   "Initialize the stream"
21   (setq *stream* stream))
22 (defun next-token ()
23   "Returns the next token of the stream"
24   (car *stream*))
25 (defun scan ()
26   (pop *stream*))
27 (defvar *stream-stack* '() "Stack of streams")
28 (defun push-stream (stream)
29   "Push the current *stream* on stack, and set this tream as *stream*"
30   (push *stream* *stream-stack*)
31   (init-stream stream))
32 (defun pop-stream ()
33   (init-stream (pop *stream-stack)))
34 (defun parse-factor ()
35   (let ((tok (next-token)))
36     (cond
37       ((or (numberp tok) (sybolp tok))
38        (scan)
39        tok)
40       ((listp tok)
41        (push-strea tok)
42        (let ((sum (parse-sum)))
43 	 (pop-stream)
44 	 (scan)
45 	 sum))
46       (t (error "Bad token in parse-atom -- ~A" tok)))))
47 (defun parse-term ()
48   (let ((lfact (parse-factor)))
49     (if (eq (next-token) '*)
50 	(progn
51 	  (scan)
52 	  (let ((rterm (parse-term)))
53 	    (list '* lfact rterm)))
54 	lfact)))
55 (defun (parse-