challenge.clj

(defn zip [xs ys]
  (lazy-seq (if (not-empty ys)
              (cons [(first xs) (first ys)]
                    (zip (rest xs) (rest ys)))
              (map (fn [i] [i nil]) xs))))


(defn f [xs]
  (map (fn [[a b]]
         (if (not (nil? b)) [a 0] [a 1]))
       (zip xs (rest xs))))


(prn (f ['a 'b 'c 'd 'e]))
(prn (f ['c]))

(prn (doseq [tuple (f (iterate inc 12312))]
       (prn tuple)
       (Thread/sleep 1000)))