> Set union can't be done efficiently on arbitrarily sized sequences. At least one of the two arguments needs to offer fast set membership; ideally the larger one, hence the requirement that all arguments be actual sets.

This made me very curious about efficient set union. Here's a pretty straightforward set union in python:

    def set_union(coll1, coll2):
        result = set()
        for item in coll1:
            result.add(item)
        for item in coll2:
            result.add(item)
        return result

Apparently that isn't the efficient way to do it -- what is?

Regardless, the article strongly suggests that one of the two arguments to clojure.set's union does offer fast set membership, since only the second argument is permitted to not be a set.