public function handle(GetResponseEvent $event)
    {
        if (null !== $this->logger) {
            $this->logger->info('Checking for guard authentication credentials.', array('firewall_key' => $this->providerKey, 'authenticators' => count($this->guardAuthenticators)));
        }

        foreach ($this->guardAuthenticators as $key => $guardAuthenticator) {
            // get a key that's unique to *this* guard authenticator
            // this MUST be the same as GuardAuthenticationProvider
            $uniqueGuardKey = $this->providerKey.'_'.$key;

            $this->executeGuardAuthenticator($uniqueGuardKey, $guardAuthenticator, $event);
        }
    }