proposition: Lemma perm_invP : left_inverse perm_one perm_inv perm_mul.
proof: Proof. by move=> s; apply/permP=> x; rewrite !permE /= permE 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)