public function setAuthClass($class)
  {
    $prev = $this->configuration['auth_class'];
    if (!isset($this->configuration['classes'][$class]) &&
        isset($this->configuration['classes'][$prev])) {
      $this->configuration['classes'][$class] =
          $this->configuration['classes'][$prev];
    }
    $this->configuration['auth_class'] = $class;
  }