proposition: Lemma same_cskip_edge {u v} : ~~ edom (val u) -> cedge u v = cedge (val u) (val v).
proof: Proof. by move=> edom'u; have [_ ->] := adj_edom. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(fintype path fingraph) fourcolor(hypermap)