proposition: Lemma domB D1 D2 : {subset dom (D1 - D2) <= (dom D1) ++ (dom D2)}.
proof: Proof. by move=> z /domD_subset; rewrite !mem_cat domN. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(fintype bigop order generic_quotient) mathcomp(ssralg ssrnum ssrint)