public function getUser($property, $value)
    {
        $class = new \core_kernel_classes_Class(GenerisRdf::CLASS_GENERIS_USER);

        $users = \tao_models_classes_UserService::singleton()->searchInstances(
            array($property => $value), 
            $class,
            array('like' => false, 'recursive' => true)
        );
        $user = empty($users) ? null : current($users);
        
        return $user;
    }