proposition: Lemma isOpen_or : ∏ A B : T → hProp, isOpen A → isOpen B → isOpen (λ x : T, A x ∨ B x).
proof: Proof. intros A B Ha Hb. rewrite <- union_or. apply isOpen_union. intros C. apply hinhuniv. apply sumofmaps ; intros ->. - exact Ha. - exact Hb. Qed.
imports: UniMath.Foundations.All UniMath.MoreFoundations.All UniMath.Topology.Filters UniMath.Algebra.Groups UniMath.Algebra.DivisionRig UniMath.Algebra.ConstructiveStructures