private function _getCriteriaKeyList( MW_Common_Criteria_Interface $criteria, array $required )
	{
		$keys = array_merge( $required, $this->_getCriteriaKeys( $required, $criteria->getConditions() ) );

		foreach( $criteria->getSortations() as $sortation ) {
			$keys = array_merge( $keys, $this->_getCriteriaKeys( $required, $sortation ) );
		}

		$keys = array_unique( array_merge( $required, $keys ) );
		sort( $keys );

		return $keys;
	}