(** Least upper bound *)

Definition nmem_lub (nm1 nm2: nmem) : nmem :=
  match nm1, nm2 with
  | NMemDead, _ => nm2
  | _, NMemDead => nm1
  | NMem stk1 gl1, NMem stk2 gl2 =>
      NMem (ISet.inter stk1 stk2)
           (PTree.combine
                (fun o1 o2 =>
                  match o1, o2 with
                  | Some iv1, Some iv2 => Some(ISet.inter iv1 iv2)
                  | _, _ => None
                  end)
                gl1 gl2)
  end.

Lemma nlive_lub_l: forall nm1 nm2 b i, nlive nm1 b i -> nlive (nmem_lub nm1 nm2) b i.