(** [sign_ext n x] is the unique integer congruent to [x] modulo [2^n] in the range [-2^(n-1)...2^(n-1) - 1]. *)

Lemma eqmod_sign_ext':
forall n x, 0 < n < zwordsize ->
eqmod (two_p n) (unsigned (sign_ext n x)) (unsigned x).