bug#57684: locked narrowing breaks existing code without an apparent way to repair

[Gregory Heytings]

> > This issue is being worked on, see the (not yet finished) 
> > feature/improved-locked-narrowing branch.
>
> Is there much to be done on that branch before merging?

I don't know yet.  Not "that much" I think, but there are a number of 
things that need to be fixed/improved.