code: func (s1 Int) Union(s2 Int) Int {
  	result := NewInt()
  	for key := range s1 {
  		result.Insert(key)
  	}
  	for key := range s2 {
  		result.Insert(key)
  	}
  	return result
  }
docs: // Union returns a new set which includes items in either s1 or s2.
  // For example:
  // s1 = {a1, a2}
  // s2 = {a3, a4}
  // s1.Union(s2) = {a1, a2, a3, a4}
  // s2.Union(s1) = {a1, a2, a3, a4}