;;; Constructor definition
(defn make-fibonacci-seq
  ([] (concat '(0 1 2)
            (fibonacci-seq 1 2)))
  ([prev-prev prev]
     (lazy-seq (let [new-number (+ prev prev-prev)]
                 (cons new-number
                       (make-fibonacci-seq prev
                                      new-number))))))

;; Creation of an instance of a fibonacci seq inside
;; a do cycle that ends up by returning nil, so that
;; the repl doesn't try to print something infinite
(do (def fb-seq (make-fibonacci-seq))
    nil)

;; Using the Infinite Seq
(first fb-seq) ;=> 0
(nth fb-seq 15) ;=> 987
(nth (fibonacci-seq) 500) ;; => ;;2255915161619363308725126950360720720460113249137581905886388664
;;18474627738686883405015987052796968498626