Генерация правильных скобочных последовательностей

(defun lb (n) (format t "~A" "(")
  (cond ((> n 1) (lb (- n 1)))))
    
(defun rb (n) (format t "~A" ")")
  (cond ((> n 1) (rb (- n 1)))))
  
(defun lrb (n)
  (format t "~A" "(")
  (format t "~A" ")")
  (cond ((> n 1) (lrb (- n 1)))))  
    
(lb 3) (rb 3) (lrb 3)

(defun task (n)
  (if (= n 1) (list "()")
      (let ((prev (task (- n 1)))
            (res nil))
        (iter (for seq in prev) (collecting (strCat "(" seq ")") into res)
                                (collecting (strCat "()" seq) into res)
                                (collecting (strcat seq "()") into res)) 
        (setof res))))
         
(task 3)
==> ("((()))" "()(())" "(())()" "(()())" "()()()")
 
(task 4)
==> ("(((())))" "()((()))" "((()))()" "(()(()))" "()()(())" "((())())" "()(())()" "(())()()" "((()()))" "()(()())" "(()())()" "(()()())" "()()()()")

(defun next-seq (seq &optional (acc nil))
  (if (null seq) (setof acc)
      (let ((curr (car seq)))
        (next-seq (cdr seq) (append (list (strCat "(" curr ")") (strCat "()" curr) (strCat curr "()")) acc)))))  
  
(defun task (n)
  (if (= n 1) (list "()")
      (let ((prev (task (- n 1))))
            (next-seq prev)))) 
 
(task 3)
==> ("(()())" "()()()" "((()))" "()(())" "(())()")
 
(task 5)
 
==> ("((()())())" "(()())()()" "(()(()()))" "()()(()())" "()(()())()" "(((()())))" "()((()()))" "((()()))()"
         "(()()()())" "()()()()()" "((()()()))" "()(()()())" "(()()())()" "(((()))())" "((()))()()" "(()((())))" 
         "()()((()))" "()((()))()" "((((()))))" "()(((())))" "(((())))()" "(()(())())" "(()()(()))" "()()()(())" 
         "()()(())()" "((()(())))" "()(()(()))" "(()(()))()" "((())()())" "()(())()()" "(())()()()" "(((())()))"
         "()((())())" "((())())()")