Sets the command manager in a special mode where all subsequential commands are tracked inside an UndoablePeriodCommand which, when undone/redone will not destroy its command queue during undo and won't accept new commands when redone, which is useful when you want to mark a method as an undoable piece of code and the method creates objects, which can be a problem with a normal command calling the method because the objects created inside the method are re-created (so you'll get new instances) when the command is redone. If follow up commands work on the instances, redoing these commands as well causes a problem as they'll work on objects which aren't there.

public void BeginUndoablePeriod(
	UndoablePeriodCommand cmd
)

Public Sub BeginUndoablePeriod ( 
	cmd As UndoablePeriodCommand
)