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