public function makeGroup($templates, $combineOperator=Criteria::OPERATOR_AND) {
    $this->groups[] = ['tpls' => $templates, self::PROPERTY_COMBINE_OPERATOR => $combineOperator];
    // store grouped nodes in an extra array to separate them from the others
    for ($i=0; $i<sizeof($templates); $i++) {
      if ($templates[$i] != null) {
        $this->groupedOIDs[] = $templates[$i]->getOID();
      }
      else {
        throw new IllegalArgumentException("Null value found in group");
      }
    }
  }