Definition lub (x y: t) : t :=
  combine
    (fun a b =>
       match a, b with
       | Some u, Some v => Some (L.lub u v)
       | None, _ => b
       | _, None => a
       end)
    x y.

(* Specifications: *)
Lemma ge_lub_left: forall x y, ge (lub x y) x.
Lemma ge_lub_right: forall x y, ge (lub x y) y.