  void extract_interior()
  { CGAL_NEF_TRACEN("extract interior");
    if ( this->is_shared() ) clone_rep();
    Overlayer D(pm());
    Vertex_iterator v, vend = D.vertices_end();
    for(v = D.vertices_begin(); v != vend; ++v)      D.mark(v) = false;
    Halfedge_iterator e, eend = D.halfedges_end();
    for(e = D.halfedges_begin(); e != eend; ++(++e)) D.mark(e) = false;
    D.simplify(Except_frame_box_edges(pm()));
  }
  void extract_interior()
  { CGAL_NEF_TRACEN("extract interior");
    if ( this->is_shared() ) clone_rep();
    Overlayer D(pm());
    Vertex_iterator v, vend = D.vertices_end();
    for(v = D.vertices_begin(); v != vend; ++v)      D.mark(v) = false;
    Halfedge_iterator e, eend = D.halfedges_end();
    for(e = D.halfedges_begin(); e != eend; ++(++e)) D.mark(e) = false;
    D.simplify(Except_frame_box_edges(pm()));
  }