sort of cons-ing of sequences

concatenation.clj

(require '[clojure.core.protocols :as p])

(deftype Concatenation [coll1 coll2]
  clojure.lang.IReduceInit
  (reduce [_ f init]
    (reduce f coll2 (reduce f init coll1 )))
  clojure.lang.Seqable
  (seq [this] (concat coll1 coll2)))

(defn concatenation
  ([coll1] coll1)
  ([coll1 coll2] (Concatenation. coll1 coll2))
  ([coll1 coll2 coll3]
   (-> coll1
       (Concatenation. coll2)
       (Concatenation. coll3)))
  ([coll1 coll2 coll3 & colls]
   (loop [c (-> coll1
                (Concatenation. coll2)
                (Concatenation. coll3))
          [coll & colls] colls]
     (if coll
       (Concatenation. c coll)
       c))))