protected function _mongo_addGroup($opt) {
		if (!$this->grp_stack)
			$this->grp_stack = array('keys'=>array(),'initial'=>array(),'reduce'=>'','finalize'=>'');
		if (isset($opt['keys']))
			$this->grp_stack['keys']+=$opt['keys'];
		if (isset($opt['reduce']))
			$this->grp_stack['reduce'].=$opt['reduce'];
		if (isset($opt['initial']))
			$this->grp_stack['initial']+=$opt['initial'];
		if (isset($opt['finalize']))
			$this->grp_stack['finalize'].=$opt['finalize'];
	}