(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 "[]][[]")