bug#9159: 24.0.50; `undo' is not as good as it should be wrt property changes

[Stefan Monnier]

>> > C-M-_ to undo the last change.
>> > With point in the same place, do `C-u C-x ='.
>> > That shows that there is property `foo' there, with value nil.
>> Do you have an example situation where the difference matters?
> What does "matters" mean?

By "matters" I mean that the difference is reflected in a different
behavior for "the end user".  I'm sure you know what I mean.

> Evidently it does not matter much to you.

Not until I see that it has some more serious consequence.  What you
show is a mostly cosmetic difference.

>> Usually we handle the absence of a text property as synonym to a nil
>> value of that property.
> Yes, I know that.  And this is a good example where it makes
> a difference.

If you accept the fact that "nothing == nil" for text-properties, then
the difference you show is indeed not a difference, it's just
a cosmetically different way to represent the same state.


        Stefan