Local Arguments nat_cast: simpl never.

Lemma wmsb_combine_existT: (* Note: not axiom free *)
forall sz (w: word sz) sz1 (w1: word sz1) sz2 (w2: word sz2) b1 b2,
sz2 <> 0 ->
existT word _ w = existT word _ (combine w1 w2) ->
wmsb w b1 = wmsb w2 b2.