code: func uniqueVars(s1, s2 []string) error {
  	for _, v1 := range s1 {
  		for _, v2 := range s2 {
  			if v1 == v2 {
  				return fmt.Errorf("mux: duplicated route variable %q", v2)
  			}
  		}
  	}
  	return nil
  }
docs: // uniqueVars returns an error if two slices contain duplicated strings.