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diff --git a/src/Util/WordUtil.v b/src/Util/WordUtil.v
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+Require Import NPeano.
+Require Import ZArith.ZArith.
+Require Import Bedrock.Word.
+
+Local Open Scope nat_scope.
+
+Lemma pow2_id : forall n, pow2 n = 2 ^ n.
+Proof.
+  induction n; intros; simpl; auto.
+Qed.
+
+Lemma Zpow_pow2 : forall n, pow2 n = Z.to_nat (2 ^ (Z.of_nat n)).
+Proof.
+  induction n; intros; auto.
+  simpl pow2.
+  rewrite Nat2Z.inj_succ.
+  rewrite Z.pow_succ_r by apply Zle_0_nat.
+  rewrite untimes2.
+  rewrite Z2Nat.inj_mul by (try apply Z.pow_nonneg; omega).
+  rewrite <- IHn.
+  auto.
+Qed.
+
+Definition wfirstn n {m} (w : Word.word m) {H : n <= m} : Word.word n.
+  refine (Word.split1 n (m - n) (match _ in _ = N return Word.word N with
+                            | eq_refl => w
+                            end)); abstract omega. Defined.