  bool is_empty() const
  /*{\Mop returns true if |\Mvar| is empty, false otherwise.}*/
  { Const_decorator D(pm());
    Face_const_iterator f = D.faces_begin();
    if(check_tag(typename Is_extended_kernel<Extended_kernel>::
                 value_type()))
      return (D.number_of_vertices()==4 &&
              D.number_of_edges()==4 &&
              D.number_of_faces()==2 &&
              D.mark(++f) == false);
    else
      return (D.number_of_vertices()==0 &&
              D.number_of_edges()==0 &&
              D.number_of_faces()==1 &&
                D.mark(f) == false);
  }
  bool is_empty() const
   
  { Const_decorator D(pm());
    Face_const_iterator f = D.faces_begin();
    if(check_tag(typename Is_extended_kernel<Extended_kernel>::
                 value_type()))
      return (D.number_of_vertices()==4 &&
              D.number_of_edges()==4 &&
              D.number_of_faces()==2 &&
              D.mark(++f) == false);
    else
      return (D.number_of_vertices()==0 &&
              D.number_of_edges()==0 &&
              D.number_of_faces()==1 &&
                D.mark(f) == false);
  }