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