title: WPF: Trigger on DependencyProperty.UnsetValue
body: <p>in an answer to <a href="https://stackoverflow.com/questions/2596847/wpf-applying-a-trigger-on-binding-failure">this question</a> the following statement was made</p>
  
  <p>"In theory it might be possible to .... trigger on {x:Static DependencyProperty.UnsetValue}, which would be much cleaner, but this doesn't appear to work in practice."</p>
  
  <p>Has anybody had any luck in doing this?</p>