code: func Subtract(s1 Set, s2 Set) Set {
  	if s1 == nil {
  		if s2 == nil {
  			return nil
  		}
  
  		return s2.New()
  	}
  	s3 := s1.Copy()
  	s3.Subtract(s2)
  	return s3
  }
docs: // Returns a new set which is the difference between s1 and s2.  s1 and s2 are
  // unmodified.