struct g: field_functor<g,point> {
  point operator() (const point& x) const { 
    return point((abs(1-x[1]) < 1e-7) ? 1 : 0, 0, 0); }
};
struct f: field_functor<f,point> {
  point operator() (const point& x) const { return point(0,0,0); }
};
