/**
  * @return form_PreferencesService
  */
 public static function getInstance()
 {
     if (self::$instance === null) {
         self::$instance = self::getServiceClassInstance(get_class());
     }
     return self::$instance;
 }
 /**
  * @return form_persistentdocument_preferences
  */
 protected function initPersistentDocument()
 {
     $document = ModuleService::getInstance()->getPreferencesDocument('form');
     return $document !== null ? $document : form_PreferencesService::getInstance()->getNewDocumentInstance();
 }