title: Programmatically set invocation property in mule
body: <p>I'm trying to set an invocation property in Mule 3.3.1 to be used in a "Choice" flow control. My flow has a transformer (extending the AbstractTransformer) and in it I'd like to set an invocation property based on certain logic. I can set it in the .mflow file:</p>
  
  <pre><code> &lt;message-properties-transformer scope="invocation" doc:name="Set Invocation Variable"&gt;
      &lt;add-message-property key="someKey" value="someValue"/&gt;
   &lt;/message-properties-transformer&gt;
  </code></pre>
  
  <p>However, I'm not sure how I would do this programmatically in my transformer. 
  My ultimate goal is to dynamically pass the operation field in a soap client based on the mule message. Any suggestions would be appreciated.</p>