Module LM := LPMap1(L).Definition opt_lub (x y: L.t) : option L.t :=
  let z := L.lub x y in
  if L.beq z L.top then None else Some z.Definition lub (x y: t) : t :=
  match x, y with
  | Bot, _ => y
  | _, Bot => x
  | Top_except m, Top_except n =>
      Top_except
        (LM.combine
           (fun a b =>
              match a, b with
              | Some u, Some v => opt_lub u v
              | _, _ => None
              end)
           m n)
  end.

(* 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.