/**
  * @static
  * @return Sabre_CardDAV_Backend_Std
  */
 public static function getInstance()
 {
     if (self::$instance == null) {
         self::$instance = new Sabre_CardDAV_Backend_Std();
     }
     return self::$instance;
 }
/**
 * @return Sabre_CardDAV_AddressBookRootFriendica
 */
function dav_createRootContactsNode()
{
    $backends = array(Sabre_CardDAV_Backend_Std::getInstance());
    foreach ($GLOBALS["CARDDAV_PRIVATE_SYSTEM_BACKENDS"] as $backendclass) {
        $backends[] = $backendclass::getInstance();
    }
    return new Sabre_CardDAV_AddressBookRootFriendica(Sabre_DAVACL_PrincipalBackend_Std::getInstance(), $backends);
}