(def atom?
  (fn [a]
    (not (seq? a))))

(println "(atom? 'a) = " (atom? 'a))
(println "(atom? '(a)) = " (atom? '(a)))
(println "")

(def null?
  (fn [a]
    (or
      (nil? a)
      (= () a))))

(println "(null? nil) = " (null? nil))
(println "(null? 'a) =  " (null? 'a))
(println "(null? '(a)) = " (null? '(a)))
(println "(null? '()) = " (null? '()))