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