title: How do I cancel and roll back part of a workflow
body: <p>I have a very long running workflow that moves video files around between video processing devices and then reports the files state to a database which is used to drive a UI</p>
  
  <p>At times the users press a button on the UI to "Accept" a file into a video storage server.  This involves copying a file from one server to another.]</p>
  
  <p>They have asked if this activity can be cancelled.</p>
  
  <p>I've looked at the wf4 documentation and I can't see a way to roll back part of a workflow.</p>
  
  <p>Is this possible and what technique should I use. </p>