protected function _validateEnumValue($arg)
	{
		parent::_validateEnumValue($arg);
		$enum = strtoupper(trim($arg));
		if($enum !== 'ASC' && $enum !== 'DESC')
		{
			throw new \InvalidArgumentException('argument has to be a string of asc or desc!');
		}
		$this->enum = $enum;
	}