proposition: Lemma classGidl x y : y \in G -> (x ^ y) ^: G = x ^: G.
proof: Proof. by move=> Gy; rewrite -class_lcoset lcoset_id. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq choice) mathcomp(fintype div path tuple bigop prime finset)