  bool operator() (Halfedge_handle e1, Halfedge_handle e2) const {
    sort_vertices<T> SORT(*this->sncp());
    if(e1->source() != e2->source())
      return SORT(e1->source(),e2->source());
    return SORT(e1->twin()->source(), e2->twin()->source());
  }
  bool operator() (Halfedge_handle e1, Halfedge_handle e2) const {
    sort_vertices<T> SORT(*this->sncp());
    if(e1->source() != e2->source())
      return SORT(e1->source(),e2->source());
    return SORT(e1->twin()->source(), e2->twin()->source());
  }