    void supporting_segment(Halfedge_handle e, IT it)
    {
      halfedge2iterator[e->opposite()] =
        halfedge2iterator[e] = it; e->mark() = true;}
    void supporting_segment(Halfedge_handle e, IT it)
    {
      halfedge2iterator[e->opposite()] =
        halfedge2iterator[e] = it; e->mark() = true;}