function enforce($Permission, $UserID = null)
	{
	if ($UserID === null)
                throw new \RbacUserNotProvidedException ("\$UserID is a required argument.");

		if (! $this->check($Permission, $UserID)) {
            header('HTTP/1.1 403 Forbidden');
            die("<strong>Forbidden</strong>: You do not have permission to access this resource.");
        }

        return true;
	}