func (u UserSet) union(o UserSet) UserSet { return UserSet(sets.String(u).Union(sets.String(o))) }
func (u UserSet) intersection(o UserSet) UserSet { return UserSet(sets.String(u).Intersection(sets.String(o))) }
// List makes a list from the set func (u UserSet) List() []string { return sets.String(u).List() }