bug#60467: 30.0.50; primitive-undo: Changes to be undone by function different from announced

[Stefan Monnier]

>>>> For the loop currently in `subr.el` I think the (cdr ptr) is OK because
>>>> we use the `setcdr` to truncate the list, so that last cons (whose `cdr`
>>>> may be nil) will be in the list included in the (apply ....) entry.
>>>
>>> Indeed, now I see it, and I think (without being 100% sure) that it's
>>> OK. It's yet another obscure point of the original code, though.
>>
>> I hope all these revelations will find their way into comments to this
>> tricky code.
>>
>
> Well, I hope this tricky code will be replaced by the non-tricky code
> I proposed, in which none of these obscurities exist... ;-)

My hope is that we can just remove the timestamp special case, in which
case we can keep the current code mostly unchanged.


        Stefan