/** Return a combiFilter for combing the global filters with 
   * the filter of the searchPart.
	* This method may be overriden for applicable logical combination of filters
	* Default implementation: PntSqlCombiFilter withParts: this getGlobalFilters
	* currently only used by custom subclasses
	* Override getGlobalFIlters on custom subclass to select applicable filters
    * and make sure all applicable filter classes are included.
	*/
	function getGlobalCombiFilter() {
		$filter = $this->getGlobalFilter();
		if (!$filter) return null;
		
		Gen::includeClass('PntSqlCombiFilter', 'pnt/db/query');
		$combi = new PntSqlCombiFilter();
		$combi->addPart($filter);
		return $combi;
	}