proposition: Lemma divgr_id K H x : x \in K -> divgr K H x = x.
proof: Proof. by move=> Kx; rewrite /divgr remgr1 // invg1 mulg1. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq div) mathcomp(choice fintype bigop finset fingroup morphism) mathcomp(quotient action)