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