code: func (s CPUSet) Union(s2 CPUSet) CPUSet {
  	b := NewBuilder()
  	for cpu := range s.elems {
  		b.Add(cpu)
  	}
  	for cpu := range s2.elems {
  		b.Add(cpu)
  	}
  	return b.Result()
  }
docs: // Union returns a new CPU set that contains all of the elements from this
  // set and all of the elements from the supplied set, without mutating
  // either source set.