Thanks for your feedback!

>
> I have one question about the new `with-restriction` macro: if it is 
> intended as a convenience macro for unlabelled narrowing as well, 
> shouldn't it then expand to the obvious
>
>  (save-restriction
>    (narrow-to-region BEG END)
>    BODY)
>
> instead the much more expensive call to a helper function?
>

Yes.  In fact now that we have a separate function for the labeled case, 
we can remove the funcall in both cases.  Patch attached.

>
> The documentation should also make it clear that `nil` is not a valid 
> label.
>

Is that not already clear in the docstring, which says "in which LABEL is 
a symbol"?