proposition: Lemma tpermC x y : tperm x y = tperm y x.
proof: Proof. by apply/permP => z; do 2![case: tpermP => //] => ->. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq path) mathcomp(choice fintype tuple finfun bigop finset binomial) mathcomp(fingroup morphism)