Local Arguments nat_cast: simpl never.

Lemma existT_wplus:
forall sz (w1 w2: word sz) sz' (w3 w4: word sz'),
existT word _ w1 = existT word _ w3 ->
existT word _ w2 = existT word _ w4 ->
existT word _ (w1 ^+ w2) = existT word _ (w3 ^+ w4).