proposition: Lemma fproperUl A B : ~~ (B `<=` A) -> A `<` A `|` B.
proof: Proof. by move=> not_sBA; rewrite fsetUC fproperUr. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)