code: func (a *OAuthStrategy) Login() error {
  	if a.AccessToken() != "" && a.AccessToken() != "revoked" {
  		return nil
  	}
  
  	return a.requestToken()
  }
docs: // Login will make a token grant request to the OAuth server
  //
  // The grant type will be password grant if Username is not empty, and client
  // credentials grant otherwise.
  //
  // On success, the AccessToken and RefreshToken (if given) will be populated.
  //
  // Login will be a no-op if the AccessToken is not empty when invoked.