code: func LogoutURL(c context.Context, dest string) (string, error) {
  	if s := GetState(c); s != nil {
  		return s.Authenticator().LogoutURL(c, dest)
  	}
  	return "", ErrNotConfigured
  }
docs: // LogoutURL returns a URL that, when visited, signs the user out, then
  // redirects the user to the URL specified by dest.
  //
  // Shortcut for GetState(c).Authenticator().LogoutURL(...).