proposition: Lemma divgrMl K B x y : x \in K -> divgr K B (x * y) = x * divgr K B y.
proof: Proof. by move=> Hx; rewrite /divgr remgrMl ?mulgA. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq div) mathcomp(choice fintype bigop finset fingroup morphism) mathcomp(quotient action)