proposition: Lemma valE_skip u v : val v = f (val u) -> v = skip u.
proof: Proof. by move=> def_v; apply: val_inj; rewrite /= -def_v z'G'. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(fintype path fingraph) fourcolor(hypermap)