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+(************************************************************************)
+(*         *   The Coq Proof Assistant / The Coq Development Team       *)
+(*  v      *   INRIA, CNRS and contributors - Copyright 1999-2018       *)
+(* <O___,, *       (see CREDITS file for the list of authors)           *)
+(*   \VV/  **************************************************************)
+(*    //   *    This file is distributed under the terms of the         *)
+(*         *     GNU Lesser General Public License Version 2.1          *)
+(*         *     (see LICENSE file for the text of the license)         *)
+(************************************************************************)
+
+(** * DecimalFacts : some facts about Decimal numbers *)
+
+Require Import Decimal.
+
+Lemma uint_dec (d d' : uint) : { d = d' } + { d <> d' }.
+Proof.
+ decide equality.
+Defined.
+
+Lemma rev_revapp d d' :
+ rev (revapp d d') = revapp d' d.
+Proof.
+ revert d'. induction d; simpl; intros; now rewrite ?IHd.
+Qed.
+
+Lemma rev_rev d : rev (rev d) = d.
+Proof.
+ apply rev_revapp.
+Qed.
+
+(** Normalization on little-endian numbers *)
+
+Fixpoint nztail d :=
+ match d with
+ | Nil => Nil
+ | D0 d => match nztail d with Nil => Nil | d' => D0 d' end
+ | D1 d => D1 (nztail d)
+ | D2 d => D2 (nztail d)
+ | D3 d => D3 (nztail d)
+ | D4 d => D4 (nztail d)
+ | D5 d => D5 (nztail d)
+ | D6 d => D6 (nztail d)
+ | D7 d => D7 (nztail d)
+ | D8 d => D8 (nztail d)
+ | D9 d => D9 (nztail d)
+ end.
+
+Definition lnorm d :=
+ match nztail d with
+ | Nil => zero
+ | d => d
+ end.
+
+Lemma nzhead_revapp_0 d d' : nztail d = Nil ->
+ nzhead (revapp d d') = nzhead d'.
+Proof.
+ revert d'. induction d; intros d' [=]; simpl; trivial.
+ destruct (nztail d); now rewrite IHd.
+Qed.
+
+Lemma nzhead_revapp d d' : nztail d <> Nil ->
+ nzhead (revapp d d') = revapp (nztail d) d'.
+Proof.
+ revert d'.
+ induction d; intros d' H; simpl in *;
+ try destruct (nztail d) eqn:E;
+  (now rewrite ?nzhead_revapp_0) || (now rewrite IHd).
+Qed.
+
+Lemma nzhead_rev d : nztail d <> Nil ->
+ nzhead (rev d) = rev (nztail d).
+Proof.
+ apply nzhead_revapp.
+Qed.
+
+Lemma rev_nztail_rev d :
+  rev (nztail (rev d)) = nzhead d.
+Proof.
+ destruct (uint_dec (nztail (rev d)) Nil) as [H|H].
+ - rewrite H. unfold rev; simpl.
+   rewrite <- (rev_rev d). symmetry.
+   now apply nzhead_revapp_0.
+ - now rewrite <- nzhead_rev, rev_rev.
+Qed.
+
+Lemma revapp_nil_inv d d' : revapp d d' = Nil -> d = Nil /\ d' = Nil.
+Proof.
+ revert d'.
+ induction d; simpl; intros d' H; auto; now apply IHd in H.
+Qed.
+
+Lemma rev_nil_inv d : rev d = Nil -> d = Nil.
+Proof.
+ apply revapp_nil_inv.
+Qed.
+
+Lemma rev_lnorm_rev d :
+  rev (lnorm (rev d)) = unorm d.
+Proof.
+ unfold unorm, lnorm.
+ rewrite <- rev_nztail_rev.
+ destruct nztail; simpl; trivial;
+  destruct rev eqn:E; trivial; now apply rev_nil_inv in E.
+Qed.
+
+Lemma nzhead_nonzero d d' : nzhead d <> D0 d'.
+Proof.
+ induction d; easy.
+Qed.
+
+Lemma unorm_0 d : unorm d = zero <-> nzhead d = Nil.
+Proof.
+ unfold unorm. split.
+ - generalize (nzhead_nonzero d).
+   destruct nzhead; intros H [=]; trivial. now destruct (H u).
+ - now intros ->.
+Qed.
+
+Lemma unorm_nonnil d : unorm d <> Nil.
+Proof.
+ unfold unorm. now destruct nzhead.
+Qed.
+
+Lemma nzhead_invol d : nzhead (nzhead d) = nzhead d.
+Proof.
+ now induction d.
+Qed.
+
+Lemma unorm_invol d : unorm (unorm d) = unorm d.
+Proof.
+ unfold unorm.
+ destruct (nzhead d) eqn:E; trivial.
+ destruct (nzhead_nonzero _ _ E).
+Qed.
+
+Lemma norm_invol d : norm (norm d) = norm d.
+Proof.
+ unfold norm.
+ destruct d.
+ - f_equal. apply unorm_invol.
+ - destruct (nzhead d) eqn:E; auto.
+   destruct (nzhead_nonzero _ _ E).
+Qed.