code: func (a *Authenticator) LoginURL(c context.Context, dest string) (string, error) {
  	if api := a.usersAPI(); api != nil {
  		return api.LoginURL(c, dest)
  	}
  	return "", ErrNoUsersAPI
  }
docs: // LoginURL returns a URL that, when visited, prompts the user to sign in,
  // then redirects the user to the URL specified by dest.
  //
  // Returns ErrNoUsersAPI if none of the authentication methods support login
  // URLs.