proposition: Fact skip_edge_subproof (u : tG') : skip_edge1 (val u) != z.
proof: Proof. case: u ifPn => x z'x /= [/eqP <- | z'ez]; first by rewrite (inj_eq edgeI). by case: ifPn => //; case: ifPn => // /eqP->; rewrite (canF_eq faceK) eq_sym. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(fintype path fingraph) fourcolor(hypermap)