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