[NTG-context] drawarrow and transparency

I have a MetaPost question concerning drawarrow withtransparency (or any other compound object). Is there some clever way to achieve uniform transparency for a compound object? For example, in the minimal example below, how can one obtain a transparent arrow without getting an X-ray vision of the arrowhead? \starttext \startMPpage drawarrow origin--(1cm,0) withtransparency(1,.5) ; drawarrow origin--(0,1cm) withtransparency(1,.5) ; \stopMPpage \stoptext Alan