  Nef_polyhedron_2<T,Items,Mark>  operator^(const Nef_polyhedron_2<T,Items,Mark>& N1) const
  { return symmetric_difference(N1); }
  Nef_polyhedron_2<T,Items,Mark>  operator^(const Nef_polyhedron_2<T,Items,Mark>& N1) const
  { return symmetric_difference(N1); }