gistfile1.clj

(defn interleave-all [c1 c2]
  (lazy-seq
   (let [s1 (seq c1) s2 (seq c2)]
     (when (or s1 s2)
       (cons (first s1) (cons (first s2)
                              (interleave-all (rest s1) (rest s2))))))))

=> (interleave [1 2 3] '[a b])
(1 a 2 b)

=> (interleave-all [1 2 3] '[a b])
(1 a 2 b 3 nil)