title: Flask cannot import enumerate? UndefinedError: 'enumerate' is undefined
body: <p>I just write this code in a HTML page.</p>
  
  <pre><code>{% for i, val in enumerate(['a', 'b', 'c']) %}
      &lt;td&gt;
          {{ val }}
      &lt;/td&gt;
  {% endfor %}
  
  UndefinedError: 'enumerate' is undefined
  </code></pre>
  
  <p>So, Flask do not support the enumerate?</p>