private function getSubset(FieldList $fields, $subset = array())
    {
        if (empty($subset)) {
            return $fields;
        }
        $subfieldlist = FieldList::create();
        foreach ($subset as $field) {
            if ($field = $fields->fieldByName($field)) {
                $subfieldlist->push($field);
            }
        }
        return $subfieldlist;
    }