public function unnatachRole( core_kernel_classes_Resource $user,  core_kernel_classes_Resource $role)
    {
    	try{
        	if (true === $this->userHasRoles($user, $role)){
        		$rolesProperty = new core_kernel_classes_Property(GenerisRdf::PROPERTY_USER_ROLES);
        		$options = array('like' => false, 'pattern' => $role->getUri());
        		$user->removePropertyValues($rolesProperty, $options);
        	}
        }
        catch (common_Exception $e){
        	$roleUri = $role->getUri();
        	$userUri = $user->getUri();
        	$msg = "An error occured while unnataching role '${roleUri}' from user '${userUri}': " . $e->getMessage();	
        }
    }