Local Arguments nat_cast: simpl never.

Lemma wlshift_combine_extz:
forall sn sl (wl: word sl) ssu (wu: word (ssu + sn)),
existT word (sl + (ssu + sn)) (wlshift (combine wl wu) sn) =
existT word (sn + (sl + ssu)) (extz (combine wl (split1 ssu _ wu)) sn).