proposition: Lemma permP s t : s =1 t <-> s = t.
proof: Proof. by split=> [| -> //]; rewrite unlock => eq_sv; apply/val_inj/ffunP. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq path) mathcomp(choice fintype tuple finfun bigop finset binomial) mathcomp(fingroup morphism)