transform nested lists into flat list: (1 3 (4 66 33 67) 23 16) => (1 3 4 66 33 67 23 16)

flat.clj

(defn flat-lists
  [ls]
  (letfn [(rec-flat [rec-list ls] 
            (cond (empty? rec-list) ls
                  (seq? (first rec-list)) (rec-flat (rest rec-list) 
                                                     (rec-flat (first rec-list) ls))
                  (not (seq? (first rec-list))) (rec-flat (rest rec-list) 
                                                           (cons (first rec-list) ls))))]
    (cond (seq? ls) (reverse (rec-flat ls '()))
          :else (list ls))))