path: root/theories/Structures/GenericMinMax.v

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(*  v      *   The Coq Proof Assistant  /  The Coq Development Team    *)
(* <O___,, *        INRIA-Rocquencourt  &  LRI-CNRS-Orsay              *)
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(*         *       GNU Lesser General Public License Version 2.1       *)
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Require Import Orders OrdersTac OrdersFacts Setoid Morphisms Basics.

(** * A Generic construction of min and max *)

(** ** First, an interface for types with [max] and/or [min] *)

Module Type HasMax (Import E:EqLe').
 Parameter Inline max : t -> t -> t.
 Parameter max_l : forall x y, y<=x -> max x y == x.
 Parameter max_r : forall x y, x<=y -> max x y == y.
End HasMax.

Module Type HasMin (Import E:EqLe').
 Parameter Inline min : t -> t -> t.
 Parameter min_l : forall x y, x<=y -> min x y == x.
 Parameter min_r : forall x y, y<=x -> min x y == y.
End HasMin.

Module Type HasMinMax (E:EqLe) := HasMax E <+ HasMin E.


(** ** Any [OrderedTypeFull] can be equipped by [max] and [min]
    based on the compare function. *)

Definition gmax {A} (cmp : A->A->comparison) x y :=
 match cmp x y with Lt => y | _ => x end.
Definition gmin {A} (cmp : A->A->comparison) x y :=
 match cmp x y with Gt => y | _ => x end.

Module GenericMinMax (Import O:OrderedTypeFull') <: HasMinMax O.

 Definition max := gmax O.compare.
 Definition min := gmin O.compare.

 Lemma ge_not_lt : forall x y, y<=x -> x<y -> False.
 Proof.
 intros x y H H'.
 apply (StrictOrder_Irreflexive x).
 rewrite le_lteq in *; destruct H as [H|H].
 transitivity y; auto.
 rewrite H in H'; auto.
 Qed.

 Lemma max_l : forall x y, y<=x -> max x y == x.
 Proof.
 intros. unfold max, gmax. case compare_spec; auto with relations.
 intros; elim (ge_not_lt x y); auto.
 Qed.

 Lemma max_r : forall x y, x<=y -> max x y == y.
 Proof.
 intros. unfold max, gmax. case compare_spec; auto with relations.
 intros; elim (ge_not_lt y x); auto.
 Qed.

 Lemma min_l : forall x y, x<=y -> min x y == x.
 Proof.
 intros. unfold min, gmin. case compare_spec; auto with relations.
 intros; elim (ge_not_lt y x); auto.
 Qed.

 Lemma min_r : forall x y, y<=x -> min x y == y.
 Proof.
 intros. unfold min, gmin. case compare_spec; auto with relations.
 intros; elim (ge_not_lt x y); auto.
 Qed.

End GenericMinMax.


(** ** Consequences of the minimalist interface: facts about [max]. *)

Module MaxLogicalProperties (Import O:TotalOrder')(Import M:HasMax O).
 Module Import T := !MakeOrderTac O.

(** An alternative caracterisation of [max], equivalent to
    [max_l /\ max_r] *)

Lemma max_spec : forall n m,
  (n < m /\ max n m == m)  \/ (m <= n /\ max n m == n).
Proof.
 intros n m.
 destruct (lt_total n m); [left|right].
 split; auto. apply max_r. rewrite le_lteq; auto.
 assert (m <= n) by (rewrite le_lteq; intuition).
 split; auto. apply max_l; auto.
Qed.

(** A more symmetric version of [max_spec], based only on [le].
    Beware that left and right alternatives overlap. *)

Lemma max_spec_le : forall n m,
 (n <= m /\ max n m == m) \/ (m <= n /\ max n m == n).
Proof.
 intros. destruct (max_spec n m); [left|right]; intuition; order.
Qed.

Instance : Proper (eq==>eq==>iff) le.
Proof. repeat red. intuition order. Qed.

Instance max_compat : Proper (eq==>eq==>eq) max.
Proof.
intros x x' Hx y y' Hy.
assert (H1 := max_spec x y). assert (H2 := max_spec x' y').
set (m := max x y) in *; set (m' := max x' y') in *; clearbody m m'.
rewrite <- Hx, <- Hy in *.
destruct (lt_total x y); intuition order.
Qed.


(** A function satisfying the same specification is equal to [max]. *)

Lemma max_unicity : forall n m p,
 ((n < m /\ p == m)  \/ (m <= n /\ p == n)) ->  p == max n m.
Proof.
 intros. assert (Hm := max_spec n m).
 destruct (lt_total n m); intuition; order.
Qed.

Lemma max_unicity_ext : forall f,
 (forall n m, (n < m /\ f n m == m)  \/ (m <= n /\ f n m == n)) ->
 (forall n m, f n m == max n m).
Proof.
 intros. apply max_unicity; auto.
Qed.

(** [max] commutes with monotone functions. *)

Lemma max_mono: forall f,
 (Proper (eq ==> eq) f) ->
 (Proper (le ==> le) f) ->
 forall x y, max (f x) (f y) == f (max x y).
Proof.
 intros f Eqf Lef x y.
 destruct (max_spec x y) as [(H,E)|(H,E)]; rewrite E;
  destruct (max_spec (f x) (f y)) as [(H',E')|(H',E')]; auto.
 assert (f x <= f y) by (apply Lef; order). order.
 assert (f y <= f x) by (apply Lef; order). order.
Qed.

(** *** Semi-lattice algebraic properties of [max] *)

Lemma max_id : forall n, max n n == n.
Proof.
 intros. destruct (max_spec n n); intuition.
Qed.

Notation max_idempotent := max_id (only parsing).

Lemma max_assoc : forall m n p, max m (max n p) == max (max m n) p.
Proof.
 intros.
 destruct (max_spec n p) as [(H,Eq)|(H,Eq)]; rewrite Eq.
 destruct (max_spec m n) as [(H',Eq')|(H',Eq')]; rewrite Eq'.
 destruct (max_spec m p); intuition; order. order.
 destruct (max_spec m n) as [(H',Eq')|(H',Eq')]; rewrite Eq'. order.
 destruct (max_spec m p); intuition; order.
Qed.

Lemma max_comm : forall n m, max n m == max m n.
Proof.
 intros.
 destruct (max_spec n m) as [(H,Eq)|(H,Eq)]; rewrite Eq.
 destruct (max_spec m n) as [(H',Eq')|(H',Eq')]; rewrite Eq'; order.
 destruct (max_spec m n) as [(H',Eq')|(H',Eq')]; rewrite Eq'; order.
Qed.

(** *** Least-upper bound properties of [max] *)

Lemma le_max_l : forall n m, n <= max n m.
Proof.
 intros; destruct (max_spec n m); intuition; order.
Qed.

Lemma le_max_r : forall n m, m <= max n m.
Proof.
 intros; destruct (max_spec n m); intuition; order.
Qed.

Lemma max_l_iff : forall n m, max n m == n <-> m <= n.
Proof.
 split. intro H; rewrite <- H. apply le_max_r. apply max_l.
Qed.

Lemma max_r_iff : forall n m, max n m == m <-> n <= m.
Proof.
 split. intro H; rewrite <- H. apply le_max_l. apply max_r.
Qed.

Lemma max_le : forall n m p, p <= max n m -> p <= n \/ p <= m.
Proof.
 intros n m p H; destruct (max_spec n m);
  [right|left]; intuition; order.
Qed.

Lemma max_le_iff : forall n m p, p <= max n m <-> p <= n \/ p <= m.
Proof.
 intros. split. apply max_le.
 destruct (max_spec n m); intuition; order.
Qed.

Lemma max_lt_iff : forall n m p, p < max n m <-> p < n \/ p < m.
Proof.
 intros. destruct (max_spec n m); intuition;
  order || (right; order) || (left; order).
Qed.

Lemma max_lub_l : forall n m p, max n m <= p -> n <= p.
Proof.
 intros; destruct (max_spec n m); intuition; order.
Qed.

Lemma max_lub_r : forall n m p, max n m <= p -> m <= p.
Proof.
 intros; destruct (max_spec n m); intuition; order.
Qed.

Lemma max_lub : forall n m p, n <= p -> m <= p -> max n m <= p.
Proof.
 intros; destruct (max_spec n m); intuition; order.
Qed.

Lemma max_lub_iff : forall n m p, max n m <= p <-> n <= p /\ m <= p.
Proof.
 intros; destruct (max_spec n m); intuition; order.
Qed.

Lemma max_lub_lt : forall n m p, n < p -> m < p -> max n m < p.
Proof.
 intros; destruct (max_spec n m); intuition; order.
Qed.

Lemma max_lub_lt_iff : forall n m p, max n m < p <-> n < p /\ m < p.
Proof.
 intros; destruct (max_spec n m); intuition; order.
Qed.

Lemma max_le_compat_l : forall n m p, n <= m -> max p n <= max p m.
Proof.
 intros.
 destruct (max_spec p n) as [(LT,E)|(LE,E)]; rewrite E.
 assert (LE' := le_max_r p m). order.
 apply le_max_l.
Qed.

Lemma max_le_compat_r : forall n m p, n <= m -> max n p <= max m p.
Proof.
 intros. rewrite (max_comm n p), (max_comm m p).
 auto using max_le_compat_l.
Qed.

Lemma max_le_compat : forall n m p q, n <= m -> p <= q ->
 max n p <= max m q.
Proof.
 intros  n m p q Hnm Hpq.
 assert (LE := max_le_compat_l _ _ m Hpq).
 assert (LE' := max_le_compat_r _ _ p Hnm).
 order.
Qed.

End MaxLogicalProperties.


(** ** Properties concernant [min], then both [min] and [max].

   To avoid too much code duplication, we exploit that [min] can be
   seen as a [max] of the reversed order.
*)

Module MinMaxLogicalProperties (Import O:TotalOrder')(Import M:HasMinMax O).
 Include MaxLogicalProperties O M.
 Import T.

 Module ORev := TotalOrderRev O.
 Module MRev <: HasMax ORev.
  Definition max x y := M.min y x.
  Definition max_l x y := M.min_r y x.
  Definition max_r x y := M.min_l y x.
 End MRev.
 Module MPRev := MaxLogicalProperties ORev MRev.

Instance min_compat : Proper (eq==>eq==>eq) min.
Proof. intros x x' Hx y y' Hy. apply MPRev.max_compat; assumption. Qed.

Lemma min_spec : forall n m,
 (n < m /\ min n m == n) \/ (m <= n /\ min n m == m).
Proof. intros. exact (MPRev.max_spec m n). Qed.

Lemma min_spec_le : forall n m,
 (n <= m /\ min n m == n) \/ (m <= n /\ min n m == m).
Proof. intros. exact (MPRev.max_spec_le m n). Qed.

Lemma min_mono: forall f,
 (Proper (eq ==> eq) f) ->
 (Proper (le ==> le) f) ->
 forall x y, min (f x) (f y) == f (min x y).
Proof.
 intros. apply MPRev.max_mono; auto. compute in *; eauto.
Qed.

Lemma min_unicity : forall n m p,
 ((n < m /\ p == n)  \/ (m <= n /\ p == m)) ->  p == min n m.
Proof. intros n m p. apply MPRev.max_unicity. Qed.

Lemma min_unicity_ext : forall f,
 (forall n m, (n < m /\ f n m == n)  \/ (m <= n /\ f n m == m)) ->
 (forall n m, f n m == min n m).
Proof. intros f H n m. apply MPRev.max_unicity, H; auto. Qed.

Lemma min_id : forall n, min n n == n.
Proof. intros. exact (MPRev.max_id n). Qed.

Notation min_idempotent := min_id (only parsing).

Lemma min_assoc : forall m n p, min m (min n p) == min (min m n) p.
Proof. intros. symmetry; apply MPRev.max_assoc. Qed.

Lemma min_comm : forall n m, min n m == min m n.
Proof. intros. exact (MPRev.max_comm m n). Qed.

Lemma le_min_r : forall n m, min n m <= m.
Proof. intros. exact (MPRev.le_max_l m n). Qed.

Lemma le_min_l : forall n m, min n m <= n.
Proof. intros. exact (MPRev.le_max_r m n). Qed.

Lemma min_l_iff : forall n m, min n m == n <-> n <= m.
Proof. intros n m. exact (MPRev.max_r_iff m n). Qed.

Lemma min_r_iff : forall n m, min n m == m <-> m <= n.
Proof. intros n m. exact (MPRev.max_l_iff m n). Qed.

Lemma min_le : forall n m p, min n m <= p -> n <= p \/ m <= p.
Proof. intros n m p H. destruct (MPRev.max_le _ _ _ H); auto. Qed.

Lemma min_le_iff : forall n m p, min n m <= p <-> n <= p \/ m <= p.
Proof. intros n m p. rewrite (MPRev.max_le_iff m n p); intuition. Qed.

Lemma min_lt_iff : forall n m p, min n m < p <-> n < p \/ m < p.
Proof. intros n m p. rewrite (MPRev.max_lt_iff m n p); intuition. Qed.

Lemma min_glb_l : forall n m p, p <= min n m -> p <= n.
Proof. intros n m. exact (MPRev.max_lub_r m n). Qed.

Lemma min_glb_r : forall n m p, p <= min n m -> p <= m.
Proof. intros n m. exact (MPRev.max_lub_l m n). Qed.

Lemma min_glb : forall n m p, p <= n -> p <= m -> p <= min n m.
Proof. intros. apply MPRev.max_lub; auto. Qed.

Lemma min_glb_iff : forall n m p, p <= min n m <-> p <= n /\ p <= m.
Proof. intros. rewrite (MPRev.max_lub_iff m n p); intuition. Qed.

Lemma min_glb_lt : forall n m p, p < n -> p < m -> p < min n m.
Proof. intros. apply MPRev.max_lub_lt; auto. Qed.

Lemma min_glb_lt_iff : forall n m p, p < min n m <-> p < n /\ p < m.
Proof. intros. rewrite (MPRev.max_lub_lt_iff m n p); intuition. Qed.

Lemma min_le_compat_l : forall n m p, n <= m -> min p n <= min p m.
Proof. intros n m. exact (MPRev.max_le_compat_r m n). Qed.

Lemma min_le_compat_r : forall n m p, n <= m -> min n p <= min m p.
Proof. intros n m. exact (MPRev.max_le_compat_l m n). Qed.

Lemma min_le_compat : forall n m p q, n <= m -> p <= q ->
 min n p <= min m q.
Proof. intros. apply MPRev.max_le_compat; auto. Qed.


(** *** Combined properties of min and max *)

Lemma min_max_absorption : forall n m, max n (min n m) == n.
Proof.
 intros.
 destruct (min_spec n m) as [(C,E)|(C,E)]; rewrite E.
 apply max_l. order.
 destruct (max_spec n m); intuition; order.
Qed.

Lemma max_min_absorption : forall n m, min n (max n m) == n.
Proof.
 intros.
 destruct (max_spec n m) as [(C,E)|(C,E)]; rewrite E.
 destruct (min_spec n m) as [(C',E')|(C',E')]; auto. order.
 apply min_l; auto. order.
Qed.

(** Distributivity *)

Lemma max_min_distr : forall n m p,
 max n (min m p) == min (max n m) (max n p).
Proof.
 intros. symmetry. apply min_mono.
 eauto with *.
 repeat red; intros. apply max_le_compat_l; auto.
Qed.

Lemma min_max_distr : forall n m p,
 min n (max m p) == max (min n m) (min n p).
Proof.
 intros. symmetry. apply max_mono.
 eauto with *.
 repeat red; intros. apply min_le_compat_l; auto.
Qed.

(** Modularity *)

Lemma max_min_modular : forall n m p,
 max n (min m (max n p)) == min (max n m) (max n p).
Proof.
 intros. rewrite <- max_min_distr.
 destruct (max_spec n p) as [(C,E)|(C,E)]; rewrite E; auto with *.
 destruct (min_spec m n) as [(C',E')|(C',E')]; rewrite E'.
 rewrite 2 max_l; try order. rewrite min_le_iff; auto.
 rewrite 2 max_l; try order. rewrite min_le_iff; auto.
Qed.

Lemma min_max_modular : forall n m p,
 min n (max m (min n p)) == max (min n m) (min n p).
Proof.
 intros. rewrite <- min_max_distr.
 destruct (min_spec n p) as [(C,E)|(C,E)]; rewrite E; auto with *.
 destruct (max_spec m n) as [(C',E')|(C',E')]; rewrite E'.
 rewrite 2 min_l; try order. rewrite max_le_iff; right; order.
 rewrite 2 min_l; try order. rewrite max_le_iff; auto.
Qed.

(** Disassociativity *)

Lemma max_min_disassoc : forall n m p,
 min n (max m p) <= max (min n m) p.
Proof.
 intros. rewrite min_max_distr.
 auto using max_le_compat_l, le_min_r.
Qed.

(** Anti-monotonicity swaps the role of [min] and [max] *)

Lemma max_min_antimono : forall f,
 Proper (eq==>eq) f ->
 Proper (le==>inverse le) f ->
 forall x y, max (f x) (f y) == f (min x y).
Proof.
 intros f Eqf Lef x y.
 destruct (min_spec x y) as [(H,E)|(H,E)]; rewrite E;
  destruct (max_spec (f x) (f y)) as [(H',E')|(H',E')]; auto.
 assert (f y <= f x) by (apply Lef; order). order.
 assert (f x <= f y) by (apply Lef; order). order.
Qed.

Lemma min_max_antimono : forall f,
 Proper (eq==>eq) f ->
 Proper (le==>inverse le) f ->
 forall x y, min (f x) (f y) == f (max x y).
Proof.
 intros f Eqf Lef x y.
 destruct (max_spec x y) as [(H,E)|(H,E)]; rewrite E;
  destruct (min_spec (f x) (f y)) as [(H',E')|(H',E')]; auto.
 assert (f y <= f x) by (apply Lef; order). order.
 assert (f x <= f y) by (apply Lef; order). order.
Qed.

End MinMaxLogicalProperties.


(** ** Properties requiring a decidable order *)

Module MinMaxDecProperties (Import O:OrderedTypeFull')(Import M:HasMinMax O).

(** Induction principles for [max]. *)

Lemma max_case_strong : forall n m (P:t -> Type),
  (forall x y, x==y -> P x -> P y) ->
  (m<=n -> P n) -> (n<=m -> P m) -> P (max n m).
Proof.
intros n m P Compat Hl Hr.
destruct (CompSpec2Type (compare_spec n m)) as [EQ|LT|GT].
assert (n<=m) by (rewrite le_lteq; auto).
apply (Compat m), Hr; auto. symmetry; apply max_r; auto.
assert (n<=m) by (rewrite le_lteq; auto).
apply (Compat m), Hr; auto. symmetry; apply max_r; auto.
assert (m<=n) by (rewrite le_lteq; auto).
apply (Compat n), Hl; auto. symmetry; apply max_l; auto.
Defined.

Lemma max_case : forall n m (P:t -> Type),
  (forall x y, x == y -> P x -> P y) ->
  P n -> P m -> P (max n m).
Proof. intros. apply max_case_strong; auto. Defined.

(** [max] returns one of its arguments. *)

Lemma max_dec : forall n m, {max n m == n} + {max n m == m}.
Proof.
 intros n m. apply max_case; auto with relations.
 intros x y H [E|E]; [left|right]; rewrite <-H; auto.
Defined.

(** Idem for [min] *)

Lemma min_case_strong : forall n m (P:O.t -> Type),
 (forall x y, x == y -> P x -> P y) ->
 (n<=m -> P n) -> (m<=n -> P m) -> P (min n m).
Proof.
intros n m P Compat Hl Hr.
destruct (CompSpec2Type (compare_spec n m)) as [EQ|LT|GT].
assert (n<=m) by (rewrite le_lteq; auto).
apply (Compat n), Hl; auto. symmetry; apply min_l; auto.
assert (n<=m) by (rewrite le_lteq; auto).
apply (Compat n), Hl; auto. symmetry; apply min_l; auto.
assert (m<=n) by (rewrite le_lteq; auto).
apply (Compat m), Hr; auto. symmetry; apply min_r; auto.
Defined.

Lemma min_case : forall n m (P:O.t -> Type),
  (forall x y, x == y -> P x -> P y) ->
  P n -> P m -> P (min n m).
Proof. intros. apply min_case_strong; auto. Defined.

Lemma min_dec : forall n m, {min n m == n} + {min n m == m}.
Proof.
 intros. apply min_case; auto with relations.
 intros x y H [E|E]; [left|right]; rewrite <- E; auto with relations.
Defined.

End MinMaxDecProperties.

Module MinMaxProperties (Import O:OrderedTypeFull')(Import M:HasMinMax O).
 Module OT := OTF_to_TotalOrder O.
 Include MinMaxLogicalProperties OT M.
 Include MinMaxDecProperties O M.
 Definition max_l := max_l.
 Definition max_r := max_r.
 Definition min_l := min_l.
 Definition min_r := min_r.
 Notation max_monotone := max_mono.
 Notation min_monotone := min_mono.
 Notation max_min_antimonotone := max_min_antimono.
 Notation min_max_antimonotone := min_max_antimono.
End MinMaxProperties.


(** ** When the equality is Leibniz, we can skip a few [Proper] precondition. *)

Module UsualMinMaxLogicalProperties
 (Import O:UsualTotalOrder')(Import M:HasMinMax O).

 Include MinMaxLogicalProperties O M.

 Lemma max_monotone : forall f, Proper (le ==> le) f ->
  forall x y, max (f x) (f y) = f (max x y).
 Proof. intros; apply max_mono; auto. congruence. Qed.

 Lemma min_monotone : forall f, Proper (le ==> le) f ->
  forall x y, min (f x) (f y) = f (min x y).
 Proof. intros; apply min_mono; auto. congruence. Qed.

 Lemma min_max_antimonotone : forall f, Proper (le ==> inverse le) f ->
  forall x y, min (f x) (f y) = f (max x y).
 Proof. intros; apply min_max_antimono; auto. congruence. Qed.

 Lemma max_min_antimonotone : forall f, Proper (le ==> inverse le) f ->
  forall x y, max (f x) (f y) = f (min x y).
 Proof. intros; apply max_min_antimono; auto. congruence. Qed.

End UsualMinMaxLogicalProperties.


Module UsualMinMaxDecProperties
 (Import O:UsualOrderedTypeFull')(Import M:HasMinMax O).

 Module P := MinMaxDecProperties O M.

 Lemma max_case_strong : forall n m (P:t -> Type),
  (m<=n -> P n) -> (n<=m -> P m) -> P (max n m).
 Proof. intros; apply P.max_case_strong; auto. congruence. Defined.

 Lemma max_case : forall n m (P:t -> Type),
  P n -> P m -> P (max n m).
 Proof. intros; apply max_case_strong; auto. Defined.

 Lemma max_dec : forall n m, {max n m = n} + {max n m = m}.
 Proof. exact P.max_dec. Defined.

 Lemma min_case_strong : forall n m (P:O.t -> Type),
  (n<=m -> P n) -> (m<=n -> P m) -> P (min n m).
 Proof. intros; apply P.min_case_strong; auto. congruence. Defined.

 Lemma min_case : forall n m (P:O.t -> Type),
  P n -> P m -> P (min n m).
 Proof. intros. apply min_case_strong; auto. Defined.

 Lemma min_dec : forall n m, {min n m = n} + {min n m = m}.
 Proof. exact P.min_dec. Defined.

End UsualMinMaxDecProperties.

Module UsualMinMaxProperties
 (Import O:UsualOrderedTypeFull')(Import M:HasMinMax O).
 Module OT := OTF_to_TotalOrder O.
 Include UsualMinMaxLogicalProperties OT M.
 Include UsualMinMaxDecProperties O M.
 Definition max_l := max_l.
 Definition max_r := max_r.
 Definition min_l := min_l.
 Definition min_r := min_r.
End UsualMinMaxProperties.


(** From [TotalOrder] and [HasMax] and [HasEqDec], we can prove
    that the order is decidable and build an [OrderedTypeFull]. *)

Module TOMaxEqDec_to_Compare
 (Import O:TotalOrder')(Import M:HasMax O)(Import E:HasEqDec O) <: HasCompare O.

 Definition compare x y :=
  if eq_dec x y then Eq
  else if eq_dec (M.max x y) y then Lt else Gt.

 Lemma compare_spec : forall x y, CompSpec eq lt x y (compare x y).
 Proof.
 intros; unfold compare; repeat destruct eq_dec; auto; constructor.
 destruct (lt_total x y); auto.
 absurd (x==y); auto. transitivity (max x y); auto.
 symmetry. apply max_l. rewrite le_lteq; intuition.
 destruct (lt_total y x); auto.
 absurd (max x y == y); auto. apply max_r; rewrite le_lteq; intuition.
 Qed.

End TOMaxEqDec_to_Compare.

Module TOMaxEqDec_to_OTF (O:TotalOrder)(M:HasMax O)(E:HasEqDec O)
 <: OrderedTypeFull
 := O <+ E <+ TOMaxEqDec_to_Compare O M E.



(** TODO: Some Remaining questions...

--> Compare with a type-classes version ?

--> Is max_unicity and max_unicity_ext really convenient to express
    that any possible definition of max will in fact be equivalent ?

--> Is it possible to avoid copy-paste about min even more ?

*)