Task:

 Generate a string with   N   opening brackets   [   and with   N   closing brackets   ],   in some arbitrary order. 
 Determine whether the generated string is balanced; that is, whether it consists entirely of pairs of opening/closing brackets (in that order), none of which mis-nest.


Examples

   (empty)      OK
   []           OK   
   [][]         OK   
   [[][]]       OK 
   ][         NOT OK
   ][][       NOT OK
   []][[]     NOT OK



(defn gen-brackets [n]
  (->> (concat (repeat n \[) (repeat n \]))
       shuffle
       (apply str ,)))
 
(defn balanced? [s]
  (loop [[first & coll] (seq s)
	 stack '()]
    (if first
      (if (= first \[)
	(recur coll (conj stack \[))
	(when (= (peek stack) \[)
	  (recur coll (pop stack))))
      (zero? (count stack)))))