proposition: Lemma subsetU A B C : (A \subset B) || (A \subset C) -> A \subset B :|: C.
proof: Proof. by rewrite -!(setCS _ A) setCU; apply: subIset. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop)