  bool operator() (SHalfloop_handle sl1, SHalfloop_handle sl2) const {
    if(sl1 == sl2) return false;
    sort_vertices<T> SORTV(*this->sncp());
    sort_facets<T> SORTF(*this->sncp());
    if(sl1->incident_sface()->center_vertex() != sl2->incident_sface()->center_vertex())
      return SORTV(sl1->incident_sface()->center_vertex(),sl2->incident_sface()->center_vertex());
    return SORTF(sl1->facet(), sl2->facet());
  }
  bool operator() (SHalfloop_handle sl1, SHalfloop_handle sl2) const {
    if(sl1 == sl2) return false;
    sort_vertices<T> SORTV(*this->sncp());
    sort_facets<T> SORTF(*this->sncp());
    if(sl1->incident_sface()->center_vertex() != sl2->incident_sface()->center_vertex())
      return SORTV(sl1->incident_sface()->center_vertex(),sl2->incident_sface()->center_vertex());
    return SORTF(sl1->facet(), sl2->facet());
  }