proposition: Lemma iter_face_subn n x : n <= arity x -> iter (arity x - n) face (iter n face x) = x.
proof: Proof. by move=> le_n_x; rewrite -iterD subnK ?iter_face_arity. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(fintype path fingraph) fourcolor(hypermap jordan)