code: func (a *Authenticator) Transport() (http.RoundTripper, error) {
  	switch useAuth, err := a.doLoginIfRequired(false); {
  	case err != nil:
  		return nil, err
  	case useAuth:
  		return a.transport, nil // token-injecting transport
  	default:
  		return a.opts.Transport, nil // original non-authenticating transport
  	}
  }
docs: // Transport optionally performs a login and returns http.RoundTripper.
  //
  // It is a high level wrapper around CheckLoginRequired() and Login() calls. See
  // documentation for LoginMode for more details.