fact: Definition isSetOfOpen_and := ∏ A B, O A → O B → O (λ x, A x ∧ B x).
imports: UniMath.Foundations.All UniMath.MoreFoundations.All UniMath.Topology.Filters UniMath.Algebra.Groups UniMath.Algebra.DivisionRig UniMath.Algebra.ConstructiveStructures