title: Combining optional block with element in twig
body: <p>In defining a base template in twig, I want to reserve an area for special notifications, using a block. It could be an extra sidebar that may contain all kinds of things (calendar, or some extra information, whatever.) The default should be emtpy, but any child template may extend and fill it. </p>
  
  <p>Now I want to have all those extensions be included in a <code>&lt;div class='special-sidebar'&gt;</code> if the block is extended, and not show anything if it is not included. Is there any way to define the containing element in the base template?</p>
  
  <pre><code>{# Base template #}    
  Content etc...
  {% block special %} {# may be overridden by child template #}{% endblock %}
  
  {# child template #}
  {% block special %} Here, the special sidebar is filled! {% endblock %}
  </code></pre>
  
  <p>The base page should show just the content:</p>
  
  <pre><code>// base template:
  Content etc...
  </code></pre>
  
  <p>And the child page:</p>
  
  <pre><code>// child template
  Content etc...
  &lt;div class"special-sidebar"&gt; Here, the special sidebar is filled! &lt;/div&gt;
  </code></pre>
  
  <p>Where and how do I put the html for this? I could define it in every child, but that means you have to remember to use the correct html each time, I'd rather set this in the base template and not bother with it. But I don't want empty elements in my page if the block is not overridden. </p>