Local Arguments nat_cast: simpl never.

Lemma extz_sext_eq_rect:
forall sz (w: word sz) n1 n2 nsz Hnsz1,
exists Hnsz2,
eq_rect (n2 + (sz + n1)) word (extz (sext w n1) n2) nsz Hnsz1 =
eq_rect (n2 + sz + n1) word (sext (extz w n2) n1) nsz Hnsz2.