proposition: Lemma perm_oneP : left_id perm_one perm_mul.
proof: Proof. by move=> s; apply/permP => x; rewrite permE /= permE. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq path) mathcomp(choice fintype tuple finfun bigop finset binomial) mathcomp(fingroup morphism)