proposition: Lemma cardsI A B : #|A :&: B| = (#|A| + #|B| - #|A :|: B|)%N.
proof: Proof. by rewrite -cardsUI addKn. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop)