protected function defineServices()
    {
        $services = [
            RegistrarContract::class => Registrar::class,
            InvoiceNotifier::class => EmailInvoiceNotifier::class,
            UserRepositoryContract::class => UserRepository::class,
            TeamRepositoryContract::class => TeamRepository::class,
        ];

        foreach ($services as $key => $value) {
            $this->app->bindIf($key, $value);
        }
    }