proposition: Lemma rcoset_sym x y : (x \in G :* y) = (y \in G :* x).
proof: Proof. by rewrite -!memV_lcosetV lcoset_sym. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq choice) mathcomp(fintype div path tuple bigop prime finset)