const Point a=(p1+p2)/2;

     const Point a=(p1+p2)/2;

     const Point b=(p2+p3)/2;

     const Point b=(p2+p3)/2;

     const Point c=(p3+p4)/2;

     const Point c=(p3+p4)/2;

     const Point d=(a+b)/2;

     const Point d=(a+b)/2;

     const Point e=(b+c)/2;

     const Point e=(b+c)/2;

     R f1=_f(Bezier3(p1,a,d,e),_d);

     R f1=_f(Bezier3(p1,a,d,e),_d);

     R f2=_f(Bezier3(e,d,c,p4),_d);

     R f2=_f(Bezier3(e,d,c,p4),_d);

     return _s(f1,f2);

     return _s(f1,f2);

   }

   }