proposition: Lemma gnode_sub1 d : gnode d - Gpoint 1 1 = gface (d - Gpoint 1 1).
proof: Proof. by rewrite /gface -!addrA oddg_add; case: oddgP. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq path) mathcomp(div choice order ssralg ssrnum ssrint intdiv) fourcolor(hypermap)