proposition: Fact skip_subproof u : skip1 (val u) != z.
proof: Proof. apply: contraNneq (valP u) => /=; case: eqP => // fu_z fz_z. by rewrite -(inj_eq injf) fu_z fz_z. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(fintype path fingraph) fourcolor(hypermap)