public function getCurrentSession()
    {
        if (is_null($this->currentSession)) {
            $this->currentSession = $this->mapper->mapFromStorage(
                $this->request($this->buildUrl(new CurrentSession()), 'GET')
            );
        }
        if (!($this->currentSession instanceof CurrentSession)) {
            $this->currentSession = null;
            throw new NotLoggedInException('Authorization required');
        }
        return $this->currentSession;
    }