proposition: Lemma setIIl A B C : A :&: B :&: C = (A :&: C) :&: (B :&: C).
proof: Proof. by rewrite setIA !(setIAC _ C) -(setIA _ C) setIid. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop)