title: Clearing NSUndoManager's Redo stack
body: <p>In my application, there are some actions I want to undo programmatically, without giving the user the option of clicking "Redo". Is there any way to clear the Redo stack of <code>NSUndoManager</code>? If not, and I were to subclass <code>NSUndoManager</code>, is there any way to get access to the redo stack in order to clear it? I didn't see any way to from the documentation.</p>
  
  <p>Alternately, is there a way to revert the changes from the current nested undo group without it populating the Redo stack? I'm already building a nested undo group.</p>