Jelle Huisman wrote:

Wolfgang Schuster wrote:

\startuseMPgraphic{columnrule}
path p ; p := unitsquare xyscaled (OverlayWidth+5mm,OverlayHeight) ;
draw (xpart(urcorner p),ypart(urcorner
p)-\the\dimexpr\noflines\lineheight\relax) -- urcorner p ;
setbounds currentpicture to p ;
\stopuseMPgraphic


Thanks Wolfgang!, this is precisely what I need. I'll wikify it. (Think 
it's time to learn some MP...)

Oeps, I have found one problem: if there is text before the columnset, like a chapter header, the text in the columnset moves down but the {columnrule} stays at the top of the page... How can I modify the code the make sure that this doesn't happen? See sample:

%sample%
\definecolumnset[example][n=3,distance=5mm]

\startuseMPgraphic{columnrule}
path p ; p := unitsquare xyscaled (OverlayWidth+5mm,OverlayHeight) ;
draw (xpart(urcorner p),ypart(urcorner
p)-\the\dimexpr\noflines\lineheight\relax) -- urcorner p ;
setbounds currentpicture to p ;
\stopuseMPgraphic
\defineoverlay[columnrule][\useMPgraphic{columnrule}]

\setupcolumnset[example][1,2][balance=yes,background=columnrule]
\setupcolumnsetlines[example][1][1][-35]
\setupcolumnsetlines[example][1][2][-35]
\setupcolumnsetlines[example][1][3][-35]

\starttext

%\input knuth % <- uncomment this to see problem

\startcolumnset[example]
\dorecurse{2}{\input knuth}
\stopcolumnset
\stoptext
%/sample%

Thanks again,

Jelle (MkII/XeTeX)