title: How to check for the presence of a session variable in Mule MEL?
body: <p>I need to check if a session variable exists.  I can check if it is present and set to a variable:</p>
  
  <pre><code>&lt;when expression="#[sessionVars['foo'] == 'true']"&gt;
  </code></pre>
  
  <p>But I want something like:</p>
  
  <pre><code>&lt;when expression="#[sessionVars['foo']]"&gt;
  -- or --
  &lt;when expression="#[Exists(sessionVars['foo'])]"&gt;
  </code></pre>
  
  <p>Since, sometimes 'foo' is not assigned a variable.</p>
  
  <p>Any ideas?</p>