  void print_plane_impl(std::ostream& out, const CGAL::Plane_3<R> p, Null_functor)
  {
    out << p.a() << " " << p.b() << " " << p.c() << " " << p.d();
  }
  void print_plane_impl(std::ostream& out, const CGAL::Plane_3<R> p, Null_functor)
  {
    out << p.a() << " " << p.b() << " " << p.c() << " " << p.d();
  }