void GeometryInstance::enabling () 
 {
   geom->used++;
   if (geom->getType() == Geometry::GROUP) {
     GeometryGroup* group = (GeometryGroup*) geom;
     for (size_t i=0; i<group->size(); i++)
       count((*group)[i],+1);
   }
   else {
     count(geom,+1);
   }
 }