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