public function getServiceManager()
    {
        $serviceManager = $this->getServiceLocator();
        if ($serviceManager instanceof ServiceManager) {
            return $serviceManager;
        }
        $msg = is_null($serviceManager)
            ? 'ServiceLocator not initialized for '.get_class($this)
            : 'Alternate service locator not compatible with getServiceManager() in ' . __CLASS__;
        throw new InvalidServiceManagerException($msg);
    }