Clojurians Log v2

it seems like clojure.set/union only returns nil when you use nil as the first arg, or for all args:

user=> (clojure.set/union)
#{}
user=> (clojure.set/union nil)
nil
user=> (clojure.set/union nil #{})
nil
user=> (clojure.set/union #{} nil)
#{}
user=> (clojure.set/union nil nil #{})
#{}
user=> (clojure.set/union nil nil nil)
nil

Maybe something for the style guide: consider if input and output could be nilable