Flatten a nested collection using Tail Recursion.

flatten.clj

(defn my-flatten
  "Flatten a nested collection using Tail Recursion."
  [coll]
  (letfn [(step [[fst & more :as coll] res]
            (if (seq coll)
              (if (coll? fst)
                (recur (concat fst more) res)
                (recur more (cons fst res)))
              (reverse res)))]
    (step coll nil)))

;; (my-flatten [1 [2 [3 [4 [5 [6 [7] 8] 9] 10] 11] 12] 13])
;; => (1 2 3 4 5 6 7 8 9 10 11 12 13)

Thanks Srid :-)