public function addScope($scope_or_scopes)
  {
    if (is_string($scope_or_scopes) && !in_array($scope_or_scopes, $this->requestedScopes)) {
      $this->requestedScopes[] = $scope_or_scopes;
    } else if (is_array($scope_or_scopes)) {
      foreach ($scope_or_scopes as $scope) {
        $this->addScope($scope);
      }
    }
  }