title: How do I change the WPF Expander Header Text when expanded or collapsed?
body: <p>Using a WPF expander, I want the Header to change from <em>See More</em> to <em>See Less</em> when the control is expanded, and back to <em>See More</em> when it's collapsed again.
  I'd prefer a WPF pure solution, rather than a C# or other code behind method.
  I get the feeling this should be easy, but I'm struggling for the right terms to get a solution via google.</p>
  
  <p>Thanks!</p>