title: How to check if an object is not of a particular type?
body: <p>I want to check if an object is not of a particular type.  I know how to check if something <em>is</em> of a particular type:</p>
  
  <pre><code>if (t is TypeA)
  {
     ...
  }
  </code></pre>
  
  <p>but </p>
  
  <pre><code>if (t isnt TypeA)
  {
     ...
  }   
  </code></pre>
  
  <p>doesn't work.    </p>