  Nef_polyhedron_2& operator=(const Nef_polyhedron_2<T,Items,Mark>& N1)
  { Base::operator=(N1); return (*this); }
  Nef_polyhedron_2& operator=(const Nef_polyhedron_2<T,Items,Mark>& N1)
  { Base::operator=(N1); return (*this); }