A simple alternative to tikz is the usage of \framed.
>
> \define[1]\oldfact
>   {\inmframed[frame=off,strut=no,leftframe=on,bottomframe=on]{#1}}
>
> \starttext
>
> \m{\oldfact{n}}
>
> \stoptext
>
> Wolfgang
>
> Thanks a lot. It should work nicely.

-- 
Respect,
Shiv Shankar Dayal