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



(define balanced-brackets-0
  []      0   -> true
  []      _   -> false
  [#\[|R] Sum -> (balanced-brackets-0 R (+ Sum 1))
  _       0   -> false
  [_  |R] Sum -> (balanced-brackets-0 R (- Sum 1)))
 
 
(define balanced-brackets
  "" -> true
  S  -> (balanced-brackets-0 (explode (INTERN S)) 0))
 
(balanced-brackets "")
 
(balanced-brackets "[]")
(balanced-brackets "[][]")
(balanced-brackets "[[][]]")
 
(balanced-brackets "][")
(balanced-brackets "][][")
(balanced-brackets "[]][[]")
 
 