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