[NTG-context] positioning of metapost figure

Dear List, The following code draws a circle at the center of the page, ignoring the "shifted" part. \startuniqueMPgraphic{concept} draw fullcircle scaled 1cm shifted (10cm,10cm); \stopuniqueMPgraphic \defineoverlay[concept][\uniqueMPgraphic{concept}] \setupbackgrounds[page][background=concept] \starttext . \stoptext How should the metapost background figure be positioned correctly? Thanks! Best, Sylvain