(fn f [pred v coll]
   (lazy-seq (cond
    (= 0 (count coll)) ()
    (= 1 (count coll)) coll
    (pred (first coll) (second coll))
      (concat (list (first coll)) (list v)
              (f pred v (rest coll)))
    :else (concat  (list (first coll))
                (f pred v (rest coll))))))