[NTG-context] Metafun manual section titles

Hello, I'm trying to make some section titles which look like the ones of the metafun manual: title and number underlined, with number in margin. I thougth it would not be too hard but I can't do it. Actually, I'm using a frame with an overlay with a \startuniqueMPgraphic to underline the title but I can't make a frame for both the section number which is in the margin and the text which is not. I tried to make the underlining of the title text extend in the margin but it's not easy because the frame background seems to be centered automatically on the frame. Maybe I should adjust some offsets but how can I know the size of the section number... Anybody can help me on that problem ? -- Romain Diss