Fill in the definition of vlist below


;;; keys is a list of atoms, alist is of the form 
;;;  ((key1 value1)(key2 value2) ...).  Returns a list of the values
;;;  corresponding to keys.  E.g. (vlist '(a 1)(b 2)(c 3)) '(c a)) returns
;;;   (3 1).  If any of the keys is not a key in alist, the value of 
;;;  vlist is the atom *missing*.  E.g. (vlist '((a 1)) '(a b)) returns
;;;  *missing*.
(define (vlist keys alist)
  (vlistr






   ))


;;; keys is a list of atoms, alist is of the form 
;;;  ((key1 value1)(key2 value2) ...).  Calls -succeed- with a
;;;  single argument, the list of values associated with the keys, in
;;;  reverse order.
;;;  if any element of keys is not a key in alist, calls -fail- instead.
(define (vlistr keys alist -succeed- -fail-)
  (vlist1 keys alist ( ) -succeed- -fail-))

(define (vlist1 keys alist result -succeed- -fail-)
  (if (null? keys)(-succeed- result)
      (cassoc (car keys) 
	      alist 
	      (lambda (val)
		(vlist1 (cdr keys)
			alist
			(cons val result)
			-succeed-
			-fail-))
	      -fail-)))

(define (cassoc key alist -succeed- -fail-)
  (if (null? alist) (-fail-)
      (if (eq? (car (car alist))
	       key)
	  (-succeed- (car (cdr (car alist))))
	  (cassoc key (cdr alist) -succeed- -fail-))))