(define (even-nums sent)
  (cond ((empty? sent) '())
	((even? (first sent))
	     (se (first sent) (even-nums (bf sent))))
	(else
	     (even-nums (bf sent)))))


(define (over-9000 sent)
  (cond ((empty? sent) '())
	((> (first sent) 9000)
	     (se (first sent) (over-9000 (bf sent))))
	(else
	    (over-9000 (bf sent)))))


(define (keep pred sent)
  (cond ((empty? sent) '())
	((pred (first sent))
	     (se (first sent) (keep pred (bf sent))))
	(else
	     (keep pred (bf sent)))))