proposition: Lemma properUr A B : ~~ (A \subset B) -> B \proper A :|: B.
proof: Proof. by rewrite properE subsetUr subUset subxx /= andbT. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop)