code: func Domain(s string) string {
  	d, err := publicsuffix.EffectiveTLDPlusOne(s)
  	if err != nil {
  		panic(errors.Wrap(err, "effective domain"))
  	}
  
  	return d
  }
docs: // Domain returns the effective domain. For example
  // the string "api.example.com" becomes "example.com",
  // while "api.example.co.uk" becomes "example.co.uk".