proposition: Lemma card_lcoset A x : #|x *: A| = #|A|.
proof: Proof. by rewrite -lcosetE (card_imset _ (mulgI _)). Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq choice) mathcomp(fintype div path tuple bigop prime finset)