title: Pyramid Chameleon Tal:condition 'Not' issue
body: <p>I am trying to display conditional text in a Pyramid Chameleon template.  Basically, checking if the dictionary key 'maxed_out_alerts' is empty (false) or has a string 'yes' in it.</p>
  
  <pre><code>&lt;p tal:condition="not:maxed_out_alerts"&gt;&lt;h3&gt;Maxed Out.&lt;/h3&gt;&lt;/p&gt;
  &lt;p tal:condition="maxed_out_alerts"&gt;&lt;h3&gt;Not Maxed Out&lt;/h3&gt;&lt;/p&gt;
  </code></pre>
  
  <p>When 'maxed_out_alerts' is an empty string,  'Maxed Out' is only displayed (correctly).  However, If 'maxed_out_alerts' contains 'yes' string both 'Maxed Out' and "Not Maxed Out' are displayed (incorrectly).</p>
  
  <p>It seems that the NOT is always evaluated to a true condition.  It should display one or the other messages not both.  What am I doing wrong?  thanks</p>