; is a number divisible by 7?
(defun divis7 (x)
  (eq (rem x 7) 0))

(defun filter (f L)
  (cond
    ((null L)
       nil)
    ((funcall f (car L))
       (filter f (cdr L)))
    (t
       (cons (car L)
             (filter f (cdr L))))))

(filter #'divis7 '(2 7 1 14 21 3 6 35 777 778))
(2 1 3 6 778)

(filter (lambda (x) (zerop (rem x 3))) 
        '(1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16))

(1 2 4 5 7 8 10 11 13 14 16)