Flatten Arbitrary Arrays in Order

Citrusbyte.clj

(ns citus-byte)
(defn flatten2
  {:static true}
  [x]
  (letfn [(flat [coll] 
                  (lazy-seq 
                   (when-let [c (seq coll)] 
                     (let [x (first c)] 
                       (if (sequential? x) 
                         (concat (flat x) (flat (rest c))) 
                         (cons x (flat (rest c))))))))]
    (if (sequential? x) (flat x) x)))
  
  ;; Testing as (flatten2 [[1,2,[3]],4])