(def atom?
  (fn [a]
    (not (seq? a))))
    
(def null?
  (fn [a]
    (or
      (nil? a)
      (= () a))))

(def lat?
  (fn [l] 
    (cond
      (null? l) true 
      (and (seq? l)
           (atom? (first l)))
             (lat? (rest l))
      true false)))

(println (lat? '(a b c)))
(println (lat? 'a))


(def member?
  (fn [a lat]
    (cond
      (null? lat) false
      true (or
        (= (first lat) a)
        (member? a (rest lat)))) ))

(println (member? 'a '(a b c)))
(println (member? 'd '(a b c)))