proposition: Lemma end0g_edge p : end0g (gedge p) = end1g p.
proof: Proof. by rewrite -gnode_face end0g_node end0g_face. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq path) mathcomp(div choice order ssralg ssrnum ssrint intdiv) fourcolor(hypermap)