proposition: Lemma eq_card_prod (A : {pred (T1 * T2)}) : A =i predT -> #|A| = #|T1| * #|T2|.
proof: Proof. exact: eq_card_trans card_prod. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(path div)