public function unsetSessionUser($sessionKey = NULL) {
		
		$sessionKey = $sessionKey ?: SessionUser::DEFAULT_KEY_NAME;
		
		$user = Session::getSessionDataBag()->get($sessionKey);
		
		if($user) {
			
			if(!$user instanceof SessionUser) {
				throw new UserException(sprintf("Session key '%s' doesn't hold a SessionUser instance.", $sessionKey));
			}
			Session::getSessionDataBag()->remove($sessionKey);
			
		}
		
		return $user;
		
	}