Changes for clojure.set/union

zkim on Sun, 26 Sep 2010
user=> (union)
#{}
user=> (union #{1 2})
#{1 2}
user=> (union #{1 2} #{2 3})
#{1 2 3}
user=> (union #{1 2} #{2 3} #{3 4})
#{1 2 3 4}
Brian Marick on Mon, 12 Jul 2010
(union) => #{}
(union #{1 2}) => #{1 2}
(union #{1 2} #{2 3}) => #{1 2 3}
(union #{1 2} #{2 3} #{3 4}) => #{1 2 3 4}