proposition: Lemma card_cosetpre : #|coset H @*^-1 L| = (#|H| * #|L|)%N.
proof: Proof. by rewrite card_morphpre ?ker_coset ?sub_im_coset. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq div) mathcomp(choice fintype prime finset fingroup morphism) mathcomp(automorphism)