> On Mar 2, 2021, at 6:50 PM, Stefan Monnier <monnier@iro.umontreal.ca> wrote:
> 
>> That means my package that constructs an undo tree from buffer-undo-list
>> cannot distinguish the two and thus cannot work incorrectly. Is it ok to map
>> one type of undo to something other than t?
> 
> I think so, yes.  It might require a few tweaks in simple.el but nothing
> major (if at all).
> 

Here is a patch, I mapped undo-in-region records to 'undo-in-region. I also fixed a bug in undo-make-selective-list: before this change, if you undo-in-region, break the undo chain, then undo-in-region again with undo-only, ulist would be set to t and it breaks at (setq undo-elt (car ulist)).

Yuan