clean函数用于删除符合条件的list元素，filter函数用于挑选符合条件的list元素

(clean symbol? ‘(1 2 d 4 f g 5 h))   → (1 2 4 5)

(filter symbol? ‘(1 2 d 4 f g 5 h))  → (d f g h)

(define (big? x) (> x 5))        → (lambda (x) (> x 5))

(clean big? ‘(1 10 3 6 4 5 11))  → (1 3 4 5)

(clean <= ‘(3 4 -6 0 2 -3 0))  → (3 4 2)

(clean (curry match ‘(a *)) ‘((a 10) (b 5) (a 3) (c 8) (a 9)))
→  ((b 5) (c 8))

(clean (fn (p) (member p ‘("." ".."))) (directory dir-path))

list删除和挑选