apply Zle_lt_trans with ((2 ^ w - 1) / 2 ^ i). apply Zdiv_le. apply lt_0_Zpow. apply le_0__Z.
cut (x < 2 ^ w). omega. apply lt_z__Z. apply Zle_lt_trans with ((2 ^ w - 1) / 2 ^ 0).
apply Zdiv_den_le. cut (0 < 2 ^ w). omega. exact lt02w. replace (2 ^ 0) with 1. omega. trivial.
apply Zle_pow_le. omega. apply le_0__Z. replace (2 ^ 0) with 1.
replace (2 ^ w - 1) with ((2 ^ w - 1) * 1). rewrite Z_div_mult. omega. omega. ring. trivial. Qed.

Lemma is_in_Z_R_uhwm : forall x y : Z_ (2 ^ w), in_Z_ (2 ^ w) (x * y / 2 ^ w).
intros x y. unfold in_Z_. split. apply Zle_0_div. apply Zmult_le_0_compat; apply le_0__Z.