Sorry, don't know what went wrong. Is it okay that I send you this link
<https://lists.gnu.org/archive/html/bug-gnu-emacs/2020-10/msg02081.html> to
my original message (which does include the code)?

On Mon, 26 Oct 2020 at 11:45, Lars Ingebrigtsen <larsi@gnus.org> wrote:

> dalanicolai@gmail.com writes:
>
> > The code above should work correctly. However when you try to use the
> > variable defined in the first two lines for the width within the
> > definition of the tabulated-list-format then no tabular list is printed
> > but instead Emacs shows an error that the width is not a number or
> > marker (number-or-marker-p).
> >
> > I guess this is not the desired behavior?
>
> Could you show the code that is failing?
>
> --
> (domestic pets only, the antidote for overdose, milk.)
>    bloggy blog: http://lars.ingebrigtsen.no
>