title: How to check if a block exists in a twig template - Symfony2
body: <p>Imagine I have something like this in my twig template </p>
  
  <pre><code>{% block posLeft %}
     -----
  {%endblock%}
  </code></pre>
  
  <p>Is there any way to check for existance of the posLeft block without calling to:</p>
  
  <pre><code>block("posLeft") 
  </code></pre>
  
  <p>And check the return value of the posBlock to varify the existance. I am a newbie in Symfony2 + Twig.</p>