(define member?
  (lambda (item ls)
    (if (null? ls)
	#f
	(or (eq? (car ls) item)
	    (member? item (cdr ls))))))

(define fact
  (lambda (x)
    (if (= x 0)
	1
	(* x (fact (- x 1))))))

;; You may use the functions and macros defined below this line,
;; but it is not necessary for you to understand them (yet!).
;; You may need to do: (require (lib "defmacro.ss")).

(define-macro trace
  (lambda (name)
    `(set! ,name (make-traceable ,name (quote ,name)))))

(define make-traceable
  (lambda (proc name)
    (letrec
      ((tab
	(lambda (n)
	  (if (eq? n 1)
	      ""
	      (string-append "|  " (tab (- n 1)))))))
      (let ((c 1))
	(lambda args
	  (display (tab c))
	  (display (cons name args))
	  (newline)
	  (set! c (+ c 1))
	  (let ((value (apply proc args)))
	    (set! c (- c 1))
	    (display (tab c))
	    (display value)
	    (newline)
	    value))))))

>(trace fact)
>(fact 5)
(fact 5)
|  (fact 4)
|  |  (fact 3)
|  |  |  (fact 2)
|  |  |  |  (fact 1)
|  |  |  |  |  (fact 0)
|  |  |  |  |  1
|  |  |  |  1
|  |  |  2
|  |  6
|  24
120
120
>

>(trace member?)
>(member? 'foo '(1 2 3 foo 4))
(member? foo (1 2 3 foo 4))
|  (member? foo (2 3 foo 4))
|  |  (member? foo (3 foo 4))
|  |  |  (member? foo (foo 4))
|  |  |  #t
|  |  #t
|  #t
#t
#t
>

>(member? 'foo '(1 2 3 4))
(member? foo (1 2 3 4))
|  (member? foo (2 3 4))
|  |  (member? foo (3 4))
|  |  |  (member? foo (4))
|  |  |  |  (member? foo ())
|  |  |  |  #f
|  |  |  #f
|  |  #f
|  #f
#f
#f
>