Definition longofwords (x y: aval) :=
  match y, x with
  | I j, I i => L(Int64.ofwords i j)
  | _, _     => ntop2 x y
  end.

Lemma longofwords_sound: forall v w x y, vmatch v x -> vmatch w y -> vmatch (Val.longofwords v w) (longofwords x y).