Undo/redo in AIR application

I've been looking at the undoManager and not understanding some basic principles, obviously.

Situation, I have an AIR desktop app and want to include Undo/Redo in the menus. When I do that, it overrides the typically expected behavior of control-z and control-y, so I assume that I have to manage that myself now. I've been able to handle cut/paste overrides by capturing the menu event and using NativeApplication.nativeApplication.copy() paste() etc, but there's nothing comparable for undo/redo. So that took me on a quest looking at the UndoManager, which appears to be my ticket for managing Spark TextInput text operations. However, if I trap the menu event for undo and look at the undoManager.canUndo() after deleting text, it's always false.

I'm missing a step somewhere. Can someone point me to an example or a better explanation of how to manage undo/redo in an AIR application, if you want to override the built-in menu items?

Thanks.