all combos of well-formed parens of size O

parens.clj

(fn [o]
	(let [f (fn q [s o c]
		(set(remove #(or (coll? %) (nil? %))
				(tree-seq
					coll?
					seq
					(let[j (fn [sep coll] (clojure.string/join sep coll))
						a (if (pos? o) (q (concat s "(") (dec o) (inc c)))
				 		b (if (pos? c) (q (concat s ")") o (dec c)))]
			 			(if (and (zero? o) (zero? c))
							(j "" s)
							[a b]))))))] (f "" o 0)))