title: Using the Symfony2 configuration class, how do I define an array node whose children don't have keys?
body: <p>Using the configuration class, how do I define an array node without numeric keys? The children of the array do not represent further configuration options. Rather, they will be a list that will not be able to be overwritten selectively, only as a whole.</p>
  
  <p>So far I have:</p>
  
  <pre><code>public function getConfigTreeBuilder()
  {
      $treeBuilder = new TreeBuilder;
      $root = $treeBuilder-&gt;root('acme_base');
  
      $root
          -&gt;children()
              -&gt;arrayNode('entities')
  
                  // Unfortunately, this doesn't work
                  -&gt;defaultValue(array(
                      'Acme\BaseBundle\Entity\DefaultEntity1',
                      'Acme\BaseBundle\Entity\DefaultEntity2',
                  ))
  
              -&gt;end()
          -&gt;end();
  
      return $treeBuilder;
  }
  </code></pre>
  
  <p>In <code>app/config.yml</code>, I want to be able to overwrite it like this:</p>
  
  <pre><code>acme_base:
    entities:
      - 'Acme\BaseBundle\Entity\AnotherEntity1'
      - 'Acme\BaseBundle\Entity\AnotherEntity2'
      - 'Acme\BaseBundle\Entity\AnotherEntity3'
      - 'Acme\BaseBundle\Entity\AnotherEntity4'
  </code></pre>