;; DOES NOT preserve original pointer
;; returns modified list

(define (filter! pred ls)
  (cond ((null? ls) '())
	((pred (car ls)) (filter! pred (cdr ls)))
	(else
	 (begin
	   (set-cdr! ls (filter! pred (cdr ls)))
	   ls))))


;; preserves original pointer, needs sentinel list
;; returns OKAY

(define (filter! pred ls)
  (cond  ((null? ls) 'okay)
	 ((null? (cdr ls)) 'okay)
	 ((not (pred (cadr ls)))
	     (filter! pred (cdr ls))
	     (set-cdr! ls (cddr ls)))
	 (else
	     (filter! pred (cdr ls))
	     (set-cdr! ls (cdr ls)))))