  Nef_polyhedron_2<T,Items,Mark> regularization() const
  /*{\Mop returns the regularized polyhedron (closure of interior).}*/
  { Nef_polyhedron_2<T,Items,Mark> res = *this;
    res.extract_regularization();
    return res;
  }
  Nef_polyhedron_2<T,Items,Mark> regularization() const
   
  { Nef_polyhedron_2<T,Items,Mark> res = *this;
    res.extract_regularization();
    return res;
  }