也可能是我眼花，在r5rs中看漏了，自己写个
(define (filter f ls)
__(if (null? ls)
______'()
______(if (f (car ls))
__________(cons (car ls)
__________________(filter f (cdr ls)))
________________(filter f (cdr ls)))))
测试
> (filter (lambda (x) (< x 3)) '(1 2 3 4 5))
(1 2)

r6rs里有filter，其实用 map 也差不多