proposition: Lemma perm_invK s : cancel (fun x => iinv (perm_onto s x)) s.
proof: Proof. by move=> x /=; rewrite f_iinv. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq path) mathcomp(choice fintype tuple finfun bigop finset binomial) mathcomp(fingroup morphism)