proposition: Lemma dprod_card A B G : A \x B = G -> (#|A| * #|B|)%N = #|G|.
proof: Proof. by case/dprodP=> [[H K -> ->] <- _]; move/TI_cardMg. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat seq div) mathcomp(choice fintype bigop finset fingroup morphism) mathcomp(quotient action)