(defun string-of-brackets (n)
  (let* ((len (* 2 n))
         (res (make-string len))
         (opening (/ len 2))
         (closing (/ len 2)))
    (dotimes (i len res)
      (setf (aref res i)
            (cond ((zerop opening) #\])
                  ((zerop closing) #\[)
                  (t (if (= (random 2) 0)
                         (progn (decf opening) #\[)
                         (progn (decf closing) #\]))))))))

(defun balancedp (string)
  (zerop (reduce (lambda (nesting bracket)
                   (ecase bracket
                     (#\] (if (= nesting 0)
                              (return-from balancedp nil)
                              (1- nesting)))
                     (#\[ (1+ nesting))))
                 string
                 :initial-value 0)))

(defun show-balanced-brackets ()
  (dotimes (i 10)
    (let ((s (string-of-brackets i)))
      (format t "~3A: ~A~%" (balancedp s) s))))