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