bug#56682: locked narrowing

[Gregory Heytings]

> Hmm, and I also don't see the change you promised in
>
>  https://debbugs.gnu.org/cgi/bugreport.cgi?bug=57207#89

Yes, this changes is focused on the "locked narrowing" issue only, which 
is complex enough.  I'll keep my promise, don't worry 😉