Used to identify the result of a call to [[UndoUnit.merge]]
The information from the specified unit was merged in and doesn't need to be added to the undo stack.
The information from the specified unit was merged and the original unit should be removed from the undo stack.
The information from the specified unit could not be merged and should be added to the undo stack.
Used to identify the result of a call to [[UndoUnit.merge]]