public function aggregate($type, $item)
	{
		$selection = $this->createSelectionInstance();

		$selection->getQueryBuilder()->importConditions($this->queryBuilder);

		$selection->select("$type($item) AS _gres");

		if (($result = $selection->fetch()) && isset($result->_gres)) {
			return $result->_gres;
		}

		return NULL;
	}