missing xml and php:

````xml
<?xml version="1.0" encoding="UTF-8" ?>
<container xmlns="http://symfony.com/schema/dic/services">
    <services>
        <service id="security.access.post_document_voter"
            class="Acme\DemoBundle\Security\Authorization\Document\PostVoter"
            public="false">
            <tag name="security.voter" />
        </service>
    </services>
</container>
````

````php
$container
    ->register('security.access.post_document_voter', 'Acme\DemoBundle\Security\Authorization\Document\PostVoter')
    ->addTag('security.voter')
;
````