public static function addWriter(Log_Interface $writer)
  {
    if (is_null(self::$_writers)) {
      self::$_writers = array();
    }
    
    $key = get_class($writer);
    if (!isset(self::$_writers[$key])) {
      self::$_writers[$key] = $writer;
      $result = true;
    } else {
      $result = false;
    }
    
    return $result;
  }