Depending on length means that the type of the longest argument is the type of the return. So this works: (clojure.set/union #{1 2 3} [2 3]) But this does no

Depending on length means that the type of the longest argument is the type of the return. So this works: (clojure.set/union #{1 2 3} [2 3]) But this does not. (clojure.set/union [1 2 3] #{2 3}) In these two cases, the ordering of arguments does not matter.