proposition: Lemma S3_inv : involutive S3f.
proof: Proof. by move=> z; apply/eqP; case: z; do 6?case. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq div) mathcomp(choice fintype tuple finfun bigop finset fingroup) mathcomp(action perm primitive_action)