proposition: Lemma fband_icpN u : {x : G | cface u (icpN x)}.
proof: Proof. case: u => [||x]. - by exists x0; rewrite cface1 /= cface_icpN. - by exists (edge (face (edge x0))); rewrite cface1r /= eqxx. by exists x; rewrite cface_icpN. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(fintype path fingraph) fourcolor(hypermap geometry color)