Definition floatofwords (x y: aval) :=
  match x, y with
  | I i, I j => F(Float.from_words i j)
  | _, _     => ntop2 x y
  end.

Lemma floatofwords_sound: forall v w x y, vmatch v x -> vmatch w y -> vmatch (Val.floatofwords v w) (floatofwords x y).