code: func (s Set) Union(other Set) Set {
  	ret := make(Set, len(s))
  	for k := range s {
  		ret[k] = struct{}{}
  	}
  	for k := range other {
  		ret[k] = struct{}{}
  	}
  	return ret
  }
docs: // Union returns a new Set which contains all element from this set, as well
  // as all elements from the other set.
  //
  // `other` must have the same underlying type as the current set, or this will
  // panic.