public function leave(Session $session)
    {
        // remove all registrations that belong to this session
        /* @var $registration \Thruway\Registration */
        foreach ($this->registrations as $i => $registration) {
            if ($registration->getSession() === $session) {
                // if this session is the callee on pending calls - error them out
                $registration->errorAllPendingCalls();
                array_splice($this->registrations, $i, 1);
            }
        }
    }