  void clear_outer_face_cycle_marks(const Tag_true&)
  { // unset all frame marks
    Decorator D(pm());
    Face_iterator f = D.faces_begin();
    D.mark(f) = false;
    Halfedge_handle e = D.holes_begin(f);
    D.set_marks_in_face_cycle(e, false);
  }
  void clear_outer_face_cycle_marks(const Tag_true&)
  {  
    Decorator D(pm());
    Face_iterator f = D.faces_begin();
    D.mark(f) = false;
    Halfedge_handle e = D.holes_begin(f);
    D.set_marks_in_face_cycle(e, false);
  }