interleave_two.clj

(defn alternate
  "Take two collections and return a lazy seq of alternating items
  from the first coll, then from the second etc until one of the
  collections is runs out of elements. Unlike interleave, the first
  collection may contain one element more than the second, which will
  be included in the resulting laz seq."
  [c1 c2]
  (lazy-seq
    (when-first [x c1]
      (cons x (alternate c2 (rest c1))))))