proposition: Lemma setCI A B : ~: (A :&: B) = ~: A :|: ~: B.
proof: Proof. by apply/setP=> x; rewrite !inE negb_and. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop)