code: func (OAuth2) RedirectState(imp Importer, ctx *SetupContext) (state string, err error) {
  	m := imp.CallbackURLParameters(ctx.AccountNode.PermanodeRef())
  	state = m.Get("state")
  	if state == "" {
  		return "", errors.New("\"state\" not found in callback parameters")
  	}
  	return state, nil
  }
docs: // RedirectState returns the "state" query parameter that should be used for the authorization
  // phase of OAuth2 authentication. This parameter contains the query component of the redirection
  // URI. See http://tools.ietf.org/html/rfc6749#section-3.1.2.2