(defun nesting (s)
  (if (zerop (length s))
      1
      (let ((stack '()))
        (loop for v across s
              do (if (char= v #\))
                     (if (or (null stack) (char/= (pop stack) #\())
                         (return-from nesting 0))
                     (push v stack)))
        (if (null stack) 1 0))))