title: Custom activity based on HandleExternalEventActivity
body: <p>I'm using <code>HandleExternalEventActivity</code> with <code>Roles</code> property and I need to check if my <b>custom</b> <code>bool RoleProfile.IsDelegatee(string userName, string workflowName, string activityName, string eventName, string roleName) </code> method finds a match. ("I'm on vacation; UserX should be able to approve 'task X' on my behalf, but not 'task Y'")</p>
  
  <p>I tried to inherit <code>HandleExternalEventActivity</code> but <code>Execute</code> method was marked as sealed.</p>
  
  <p>Do you have any ideas?</p>
  
  <p>TIA</p>