title: form _self removes global variable
body: <pre><code>{% set admin = true %}
  
  {% extends admin ? "AcmeAdminBundle::admin.html.twig" : "AcmeUserBundle::user.html.twig" %}
  
  {% form_theme form _self %}
  
  //custom stuff
  
  &lt;form action="{{ path('process_form')}}" method="post" {{ form_enctype(form) }}&gt;
  //...whatever
  &lt;/form&gt;
  </code></pre>
  
  <p>It looks like <code>{% form_theme form _self %}</code> removes global variables as I get the following error:
  "<code>Variable "admin" does not exist</code>"</p>
  
  <p>What should I do to extend the template properly in that case...?</p>