proposition: Lemma properUl A B : ~~ (B \subset A) -> A \proper A :|: B.
proof: Proof. by move=> not_sBA; rewrite setUC properUr. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop)