public function getOneUser($login,  core_kernel_classes_Class $class = null)
    {
        $returnValue = null;

    	if(empty($class)){
        	$class = new core_kernel_classes_Class(GenerisRdf::CLASS_GENERIS_USER);
    	}
        
    	$users = $class->searchInstances(
    		array(GenerisRdf::PROPERTY_USER_LOGIN => $login), 
    		array('like' => false, 'recursive' => true)
    	);
    	
    	if (count($users) == 1){
    		$returnValue = current($users);	
    	}
    	else if (count($users) > 1){
    		$msg = "More than one user have the same login '${login}'.";
    	}

        return $returnValue;
    }