proposition: Lemma mem_rcoset A x y : (y \in A :* x) = (y * x^-1 \in A).
proof: Proof. by rewrite -rcosetE [_ x](can_imset_pre A (mulgK _)) inE. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq choice) mathcomp(fintype div path tuple bigop prime finset)