Definition elt := X.t.Definition t := list elt.Definition empty : t := nil.Definition is_empty (l : t) := if l then true else false.(** ** The set operations. *)

  Fixpoint mem x s :=
    match s with
    | nil => false
    | y :: l =>
        match X.compare x y with
        | Lt => false
        | Eq => true
        | Gt => mem x l
        end
    end.Fixpoint add x s :=
    match s with
    | nil => x :: nil
    | y :: l =>
        match X.compare x y with
        | Lt => x :: s
        | Eq => s
        | Gt => y :: add x l
        end
    end.Definition singleton (x : elt) := x :: nil.Fixpoint remove x s : t :=
    match s with
    | nil => nil
    | y :: l =>
        match X.compare x y with
        | Lt => s
        | Eq => l
        | Gt => y :: remove x l
        end
    end.Fixpoint union (s : t) : t -> t :=
    match s with
    | nil => fun s' => s'
    | x :: l =>
        (fix union_aux (s' : t) : t :=
           match s' with
           | nil => s
           | x' :: l' =>
               match X.compare x x' with
               | Lt => x :: union l s'
               | Eq => x :: union l l'
               | Gt => x' :: union_aux l'
               end
           end)
    end.Fixpoint inter (s : t) : t -> t :=
    match s with
    | nil => fun _ => nil
    | x :: l =>
        (fix inter_aux (s' : t) : t :=
           match s' with
           | nil => nil
           | x' :: l' =>
               match X.compare x x' with
               | Lt => inter l s'
               | Eq => x :: inter l l'
               | Gt => inter_aux l'
               end
           end)
    end.Fixpoint diff (s : t) : t -> t :=
    match s with
    | nil => fun _ => nil
    | x :: l =>
        (fix diff_aux (s' : t) : t :=
           match s' with
           | nil => s
           | x' :: l' =>
               match X.compare x x' with
               | Lt => x :: diff l s'
               | Eq => diff l l'
               | Gt => diff_aux l'
               end
           end)
    end.Fixpoint equal (s : t) : t -> bool :=
    fun s' : t =>
    match s, s' with
    | nil, nil => true
    | x :: l, x' :: l' =>
        match X.compare x x' with
        | Eq => equal l l'
        | _ => false
        end
    | _, _ => false
    end.Fixpoint subset s s' :=
    match s, s' with
    | nil, _ => true
    | x :: l, x' :: l' =>
        match X.compare x x' with
        | Lt => false
        | Eq => subset l l'
        | Gt => subset s l'
        end
    | _, _ => false
    end.Definition fold (B : Type) (f : elt -> B -> B) (s : t) (i : B) : B :=
    fold_left (flip f) s i.Fixpoint filter (f : elt -> bool) (s : t) : t :=
    match s with
    | nil => nil
    | x :: l => if f x then x :: filter f l else filter f l
    end.Fixpoint for_all (f : elt -> bool) (s : t) : bool :=
    match s with
    | nil => true
    | x :: l => if f x then for_all f l else false
    end.Fixpoint exists_ (f : elt -> bool) (s : t) : bool :=
    match s with
    | nil => false
    | x :: l => if f x then true else exists_ f l
    end.Fixpoint partition (f : elt -> bool) (s : t) : t * t :=
    match s with
    | nil => (nil, nil)
    | x :: l =>
        let (s1, s2) := partition f l in
        if f x then (x :: s1, s2) else (s1, x :: s2)
    end.Definition cardinal (s : t) : nat := length s.Definition elements (x : t) : list elt := x.Definition min_elt (s : t) : option elt :=
    match s with
    | nil => None
    | x :: _ => Some x
    end.Fixpoint max_elt (s : t) : option elt :=
    match s with
    | nil => None
    | x :: nil => Some x
    | _ :: l => max_elt l
    end.Definition choose := min_elt.Fixpoint compare s s' :=
   match s, s' with
    | nil, nil => Eq
    | nil, _ => Lt
    | _, nil => Gt
    | x::s, x'::s' =>
      match X.compare x x' with
       | Eq => compare s s'
       | Lt => Lt
       | Gt => Gt
      end
   end.Module Import MX := OrderedTypeFacts X.Module Import ML := OrderedTypeLists X.Definition inf x l :=
   match l with
   | nil => true
   | y::_ => match X.compare x y with Lt => true | _ => false end
   end.Fixpoint isok l :=
   match l with
   | nil => true
   | x::l => inf x l && isok l
   end.Notation Sort l := (isok l = true).Notation Inf := (lelistA X.lt).Notation In := (InA X.eq).Definition IsOk s := Sort s.Class Ok (s:t) : Prop := ok : Sort s.Class Ok (s:t) : Prop := ok : Sort s.Instance Sort_Ok s `(Hs : Sort s) : Ok s := { ok := Hs }.

Lemma isok_iff : forall l, sort X.lt l <-> Ok l.