Inductive dio_op := do_add | do_mul.Inductive dio_op := do_add | do_mul.Inductive dio_op := do_add | do_mul.Definition do_eval o :=
    match o with
      | do_add => plus
      | do_mul => mult
    end.Inductive dio_expression : Set :=
    | de_cst  : nat -> dio_expression
    | de_var  : nat -> dio_expression
    | de_comp : dio_op -> dio_expression -> dio_expression -> dio_expression.Inductive dio_expression : Set :=
    | de_cst  : nat -> dio_expression
    | de_var  : nat -> dio_expression
    | de_comp : dio_op -> dio_expression -> dio_expression -> dio_expression.Inductive dio_expression : Set :=
    | de_cst  : nat -> dio_expression
    | de_var  : nat -> dio_expression
    | de_comp : dio_op -> dio_expression -> dio_expression -> dio_expression.Inductive dio_expression : Set :=
    | de_cst  : nat -> dio_expression
    | de_var  : nat -> dio_expression
    | de_comp : dio_op -> dio_expression -> dio_expression -> dio_expression.Definition de_add := de_comp do_add.Definition de_mul := de_comp do_mul.Fixpoint de_size e :=
    match e with
      | de_cst n => 1
      | de_var x => 1
      | de_comp _ p q => 1 + de_size p + de_size q
    end.Fixpoint de_size_Z e :=
    (match e with
      | de_cst n => 1
      | de_var x => 1
      | de_comp _ p q => 1 + de_size_Z p + de_size_Z q
    end)%Z.Fact de_size_Z_spec e : de_size_Z e = Z.of_nat (de_size e).Fixpoint de_eval ν e  :=
    match e with
      | de_cst n => n
      | de_var x => ν x
      | de_comp o p q => do_eval o (de_eval ν p) (de_eval ν q)
    end.Fact de_eval_ext e ν ω : (forall x, ν x = ω x) -> de_eval ν e = de_eval ω e.(* ρ σ ν *)

  Fixpoint de_subst σ e :=
    match e with
      | de_cst n => de_cst n
      | de_var x => σ x
      | de_comp o p q => de_comp o (de_subst σ p) (de_subst σ q)
    end.Fact de_eval_subst σ ν e : de_eval ν (de_subst σ e) = de_eval (fun x => de_eval ν (σ x)) e.Fact de_subst_subst σ1 σ2 e : de_subst σ1 (de_subst σ2 e) = de_subst (fun x => de_subst σ1 (σ2 x)) e.Definition de_ren ρ := de_subst (fun x => de_var (ρ x)).Fact de_ren_size ρ e : de_size (de_ren ρ e) = de_size e.Fact de_ren_size_Z ρ e : de_size_Z (de_ren ρ e) = de_size_Z e.Fact de_eval_ren ρ ν e : de_eval ν (de_ren ρ e)  = de_eval (fun x => ν (ρ x)) e.Definition de_lift := de_ren S.Fact de_eval_lift ν e : de_eval ν (de_lift e) = de_eval (fun x => ν (S x)) e.Definition dio_expr t := { e | forall ν, de_eval ν e = t ν }.Notation 𝔻P := dio_expr.(* How to analyse meta-level diophantine expressions *)

  Implicit Types r t : (nat -> nat) -> nat.Fact dio_expr_var i : 𝔻P (fun v => v i).Fact dio_expr_cst c : 𝔻P (fun _ => c).Fact dio_expr_plus r t : 𝔻P r -> 𝔻P t -> 𝔻P (fun ν => r ν + t ν).Fact dio_expr_mult r t : 𝔻P r -> 𝔻P t -> 𝔻P (fun ν => r ν * t ν).Fact dio_expr_ren t ρ : 𝔻P t -> 𝔻P (fun ν => t (fun i => ν (ρ i))).Fact dio_expr_subst t σ : 𝔻P t -> 𝔻P (fun ν => t (fun i => de_eval ν (σ i))).(* De Bruin syntax for diophantine formulas of the form

         A,B ::= e1 = e2 | A /\ B | A \/ B | ∃x.A

   *)

  Inductive dio_formula : Set :=
    | df_atm  : dio_expression -> dio_expression -> dio_formula   (* a = b *)
    | df_conj : dio_formula -> dio_formula -> dio_formula 
    | df_disj : dio_formula -> dio_formula -> dio_formula
    | df_exst : dio_formula -> dio_formula.(* De Bruin syntax for diophantine formulas of the form

         A,B ::= e1 = e2 | A /\ B | A \/ B | ∃x.A

   *)

  Inductive dio_formula : Set :=
    | df_atm  : dio_expression -> dio_expression -> dio_formula   (* a = b *)
    | df_conj : dio_formula -> dio_formula -> dio_formula 
    | df_disj : dio_formula -> dio_formula -> dio_formula
    | df_exst : dio_formula -> dio_formula.(* De Bruin syntax for diophantine formulas of the form

         A,B ::= e1 = e2 | A /\ B | A \/ B | ∃x.A

   *)

  Inductive dio_formula : Set :=
    | df_atm  : dio_expression -> dio_expression -> dio_formula   (* a = b *)
    | df_conj : dio_formula -> dio_formula -> dio_formula 
    | df_disj : dio_formula -> dio_formula -> dio_formula
    | df_exst : dio_formula -> dio_formula.(* De Bruin syntax for diophantine formulas of the form

         A,B ::= e1 = e2 | A /\ B | A \/ B | ∃x.A

   *)

  Inductive dio_formula : Set :=
    | df_atm  : dio_expression -> dio_expression -> dio_formula   (* a = b *)
    | df_conj : dio_formula -> dio_formula -> dio_formula 
    | df_disj : dio_formula -> dio_formula -> dio_formula
    | df_exst : dio_formula -> dio_formula.(* De Bruin syntax for diophantine formulas of the form

         A,B ::= e1 = e2 | A /\ B | A \/ B | ∃x.A

   *)

  Inductive dio_formula : Set :=
    | df_atm  : dio_expression -> dio_expression -> dio_formula   (* a = b *)
    | df_conj : dio_formula -> dio_formula -> dio_formula 
    | df_disj : dio_formula -> dio_formula -> dio_formula
    | df_exst : dio_formula -> dio_formula.Fixpoint df_size f :=
    match f with
      | df_atm a b  => 1 + de_size a + de_size b
      | df_conj f g => 1 + df_size f + df_size g  
      | df_disj f g => 1 + df_size f + df_size g  
      | df_exst f   => 1 + df_size f
    end.Fixpoint df_size_Z f :=
    (match f with
      | df_atm a b  => 1 + de_size_Z a + de_size_Z b
      | df_conj f g => 1 + df_size_Z f + df_size_Z g  
      | df_disj f g => 1 + df_size_Z f + df_size_Z g  
      | df_exst f   => 1 + df_size_Z f
    end)%Z.Fact df_size_Z_spec f : df_size_Z f = Z.of_nat (df_size f).(* dv_lift : lifting of a diophantive valuation *)

  Definition dv_lift X ν (x : X) n :=
     match n with 
       | 0   => x 
       | S n => ν n 
     end.Fixpoint df_pred f ν :=
    match f with
      | df_atm a b  => de_eval ν a  = de_eval ν b
      | df_conj f g => df_pred f ν /\ df_pred g ν
      | df_disj f g => df_pred f ν \/ df_pred g ν
      | df_exst f   => exists n, df_pred f (dv_lift ν n)
    end.Fact df_pred_atm a b ν : df_pred (df_atm a b) ν = (de_eval ν a = de_eval ν b).Fact df_pred_conj f g ν : df_pred (df_conj f g) ν = (df_pred f ν /\ df_pred g ν).Fact df_pred_disj f g ν : df_pred (df_disj f g) ν = (df_pred f ν \/ df_pred g ν).Fact df_pred_exst f ν : df_pred (df_exst f) ν = exists n, df_pred f (dv_lift ν n).Fact df_pred_ext f ν ω : (forall x, ν x = ω x) -> df_pred f ν <-> df_pred f ω.(* Lifting of a diophantine expression renaming *)

  Definition der_lift ρ x := match x with 0 => 0 | S x => S (ρ x) end.Fixpoint df_ren ρ f :=
    match f with
      | df_atm a b  => let σ := fun x => de_var (ρ x) in df_atm (de_subst σ a) (de_subst σ b)
      | df_conj f g => df_conj (df_ren ρ f) (df_ren ρ g)
      | df_disj f g => df_disj (df_ren ρ f) (df_ren ρ g)
      | df_exst f   => df_exst (df_ren (der_lift ρ) f)
    end.Fact df_ren_size ρ f : df_size (df_ren ρ f) = df_size f.Fact df_ren_size_Z ρ f : df_size_Z (df_ren ρ f) = df_size_Z f.Fact df_pred_ren f ν ρ : df_pred (df_ren ρ f) ν <-> df_pred f (fun x => ν (ρ x)).(* Lifting of a diophantine expression substitutions *)

  Definition des_lift σ x := match x with 0 => de_var 0 | S x => de_ren S (σ x) end.Fixpoint df_subst σ f := 
    match f with
      | df_atm a b  => df_atm (de_subst σ a) (de_subst σ b)
      | df_conj f g => df_conj (df_subst σ f) (df_subst σ g)
      | df_disj f g => df_disj (df_subst σ f) (df_subst σ g)
      | df_exst f   => df_exst (df_subst (des_lift σ) f)
    end.Fact df_pred_subst f ν σ : df_pred (df_subst σ f) ν <-> df_pred f (fun x => de_eval ν (σ x)).Definition df_lift := df_ren S.Fact df_pred_lift f ν : df_pred (df_lift f) ν <-> df_pred f (fun x => ν (S x)).Definition df_true := df_atm (de_cst 0) (de_cst 0).Definition df_false := df_atm (de_cst 0) (de_cst 1).Fact df_true_spec : df_pred df_true ν <-> True.Fact df_false_spec : df_pred df_false ν <-> False.Notation "'⟦' x '⟧'" := (de_eval ν x).Definition df_le x y := df_exst (df_atm (de_add (de_var 0) (de_lift x)) (de_lift y)).Fact df_le_spec x y : df_pred (df_le x y) ν <-> ⟦x⟧ <= ⟦y⟧.Definition df_lt x y := df_exst (df_atm (de_add (de_cst 1) (de_add (de_var 0) (de_lift x))) (de_lift y)).Fact df_lt_spec x y : df_pred (df_lt x y) ν <-> ⟦x⟧ < ⟦y⟧.Definition df_eq x y := df_atm x y.Fact df_eq_spec x y : df_pred (df_eq x y) ν <-> ⟦x⟧ = ⟦y⟧.Definition df_neq x y := df_disj (df_lt x y) (df_lt y x).Fact df_neq_spec x y : df_pred (df_neq x y) ν <-> ⟦x⟧ <> ⟦y⟧.Definition df_div x y := df_exst (df_atm (de_lift y) (de_mul (de_var 0) (de_lift x))).Fact df_div_spec x y : df_pred (df_div x y) ν <-> divides ⟦x⟧ ⟦y⟧.Definition dio_rel R := { f | forall ν, df_pred f ν <-> R ν }.Notation 𝔻R := dio_rel.(** How to analyse diophantine relations ... these are proved by
      explicitely given the witness which we will avoid later on *)
  
  Implicit Types R S : (nat -> nat) -> Prop.Fact dio_rel_True : 𝔻R (fun _ => True).Fact dio_rel_False : 𝔻R (fun _ => False).Fact dio_rel_eq r t : 𝔻P r -> 𝔻P t -> 𝔻R (fun ν => r ν = t ν).Fact dio_rel_le r t : 𝔻P r -> 𝔻P t -> 𝔻R (fun ν => r ν <= t ν).Fact dio_rel_lt r t : 𝔻P r -> 𝔻P t -> 𝔻R (fun ν => r ν < t ν).Fact dio_rel_neq r t : 𝔻P r -> 𝔻P t -> 𝔻R (fun ν => r ν <> t ν).Fact dio_rel_div r t : 𝔻P r -> 𝔻P t -> 𝔻R (fun ν => divides (r ν) (t ν)).Fact dio_rel_conj R S : 𝔻R R -> 𝔻R S -> 𝔻R (fun ν => R ν /\ S ν).Fact dio_rel_disj R S : 𝔻R R -> 𝔻R S -> 𝔻R (fun ν => R ν \/ S ν).Fact dio_rel_exst (K : nat -> (nat -> nat) -> Prop) : 
                   𝔻R (fun v => K (v 0) (fun n => v (S n))) 
      -> 𝔻R (fun ν => exists x, K x ν).

Lemma dio_rel_subst R f : 𝔻R R -> 𝔻R (fun v => R (fun n => de_eval v (f n))).