Normalization of Simply Typed Lambda-Calculus through Hereditary Substitutions.

Uses extrinsic encoding of terms, with de Bruijn indices, lifting and substitution. Derive hereditary substitution function justified by a well-founded order on typable terms and conclude with a normalizer building beta-short eta-long normal forms, typable in a bidirectional type system.

Require Program.
Require Import Equations.Equations.
Require Import Lia.
Require Import List Utf8.

Import ListNotations.

Set Keyed Unification.

Derive Signature for le CompareSpec.

Inductive term :=
| Var (n : nat)
| Lambda (t : term)
| App (t u : term)
| Pair (t u : term)
| Fst (t : term) | Snd (t : term)
| Tt.

Derive NoConfusion Subterm EqDec for term.

Coercion Var : nat >-> term.

Declare Scope term_scope.
Delimit Scope term_scope with term.
Bind Scope term_scope with term.

Notation " @( f , x ) " := (App (f%term) (x%term)).
Notation " 'λ' t " := (Lambda (t%term)) (at level 0).
Notation " << t , u >> " := (Pair (t%term) (u%term)).

Parameter atomic_type : Set.
Parameter atomic_type_eqdec : EqDec atomic_type.
Existing Instance atomic_type_eqdec.

Inductive type :=
| atom (a : atomic_type)
| product (a b : type)
| unit
| arrow (a b : type).

Derive NoConfusion Subterm EqDec for type.

Coercion atom : atomic_type >-> type.
Notation " x × y " := (product x y) (at level 90).
Notation " x ---> y " := (arrow x y) (at level 30).

Require Import Arith.

Equations lift (k n : nat) (t : term) : term :=
lift k n (Var i) with Nat.compare i k := {
  | Lt := Var i ;
  | _ := Var (i + n) } ;
lift k n (Lambda t) := Lambda (lift (S k) n t) ;
lift k n (App t u) := @(lift k n t, lift k n u) ;
lift k n (Pair t u) := << lift k n t, lift k n u >> ;
lift k n (Fst t) := Fst (lift k n t) ;
lift k n (Snd t) := Snd (lift k n t) ;
lift k n Tt := Tt.

Tactic Notation "absurd" tactic(tac) := elimtype False; tac.

Ltac term_eq :=
  match goal with
    | |- Var _ = Var _f_equal ; lia
    | |- @eq nat _ _lia || absurd lia
    | |- lt _ _lia || absurd lia
    | |- le _ _lia || absurd lia
    | |- gt _ _lia || absurd lia
    | |- ge _ _lia || absurd lia
  end.

#[local] Hint Extern 4 ⇒ term_eq : term.

Ltac term := typeclasses eauto with term core arith.

Ltac do_rewrites :=
  repeat
    match goal with
        H : ?lhs = ?rhs |- context [?lhs] ⇒ rewrite H; clear H
    end.

Ltac crush := do_rewrites; auto; try term.

Lemma lift0 k t : lift k 0 t = t.
Proof.
  funelim (lift k 0 t); term || rewrite ?H; crush.
Qed.
#[local] Hint Rewrite lift0 : lift.
Require Import Lia.

Lemma lift_k_lift_k k n m t : lift k n (lift k m t) = lift k (n + m) t.
Proof.
  funelim (lift k m t); intros; simp lift; try rewrite H ; try rewrite H0; auto.

  destruct (Nat.compare_spec n0 k); try discriminate. subst.
  case_eq (Nat.compare (k + n) k); intro H; simp lift; try term.
  rewrite Nat.compare_lt_iff in H; term.
  rewrite Heq; simp lift; term.

  rewrite Heq. rewrite Nat.compare_gt_iff in Heq. simp lift.
  destruct (Nat.compare_spec (n0 + n) k); try discriminate; simp lift; term.
Qed.
#[local] Hint Rewrite lift_k_lift_k : lift.

Equations subst (k : nat) (t : term) (u : term) : term :=
subst k (Var i) u with Nat.compare i k := {
  | Eq := lift 0 k u ;
  | Lt := i ;
  | Gt := Var (pred i) } ;
subst k (Lambda t) u := Lambda (subst (S k) t u) ;
subst k (App a b) u := @(subst k a u, subst k b u) ;
subst k (Pair a b) u := << subst k a u, subst k b u >> ;
subst k (Fst t) u := Fst (subst k t u) ;
subst k (Snd t) u := Snd (subst k t u) ;
subst k Tt _ := Tt.

Lemma substnn n t : subst n n t = lift 0 n t.
Proof. funelim (subst n n t) ; try rewrite H ; try rewrite H0; simp lift; auto.
  rewrite Nat.compare_lt_iff in Heq; absurd lia.
  rewrite Nat.compare_gt_iff in Heq; absurd lia.
Qed.
#[local] Hint Rewrite substnn : subst.
Notation ctx := (list type).

Reserved Notation " Γ |-- t : A " (at level 70, t, A at next level).

Inductive types : ctx term type Prop :=
| axiom Γ i : i < length Γ (Γ |-- i : nth i Γ unit)

| abstraction Γ A B t :
  A :: Γ |-- t : B Γ |-- λ t : A ---> B

| application Γ A B t u :
  Γ |-- t : A ---> B Γ |-- u : A Γ |-- @(t, u) : B

| unit_intro Γ : Γ |-- Tt : unit

| pair_intro Γ A B t u :
  Γ |-- t : A Γ |-- u : B
    Γ |-- << t , u >> : (A × B)

| pair_elim_fst Γ A B t : Γ |-- t : (A × B) Γ |-- Fst t : A

| pair_elim_snd Γ A B t : Γ |-- t : (A × B) Γ |-- Snd t : B

where "Γ |-- i : A " := (types Γ i A).

Derive Signature for types.

Notation " [ t ] u " := (subst 0 u t) (at level 10).

Notation " x @ y " := (app x y) (at level 30, right associativity).

Lemma nth_length {A} x t (l l' : list A) : nth (length l) (l @ (t :: l')) x = t.
Proof. induction l; simpl; auto. Qed.

#[local] Hint Constructors types : term.

Lemma nat_compare_elim (P : nat nat comparison Prop)
  (PEq : i, P i i Eq)
  (PLt : i j, i < j P i j Lt)
  (PGt : i j, i > j P i j Gt) :
   i j, P i j (Nat.compare i j).
Proof. intros. case (Nat.compare_spec i j); intros; subst; auto. Qed.

Lemma nth_extend_left {A} (a : A) n (l l' : list A) : nth n l a = nth (length l' + n) (l' @ l) a.
Proof. induction l'; auto. Qed.

Lemma nth_app_l {A} (a : A) {n} (l l' : list A) : n < length l nth n (l @ l') a = nth n l a.
Proof.
  revert l l' n; induction l; intros; auto. depelim H. destruct n; trivial.
  simpl. eapply IHl. simpl in H. lia.
Qed.

Lemma nth_app_r {A} (a : A) {n} (l l' : list A) : length l n
  nth n (l @ l') a = nth (n - length l) l' a.
Proof.
  revert l l' n; induction l; intros; auto. simpl in H. depelim H; auto.
  destruct n; simpl in H. depelim H. simpl; apply IHl; lia.
Qed.

Lemma nth_extend_middle {A} (a : A) n (l l' l'' : list A) :
  match Nat.compare n (length l') with
    | Ltnth n (l' @ l) a = nth n (l' @ l'' @ l) a
    | _nth n (l' @ l) a = nth (n + length l'') (l' @ l'' @ l) a
  end.
Proof.
  assert (foo:=Nat.compare_spec n (length l')).
  depelim foo; fold (length l') in H;
  try rewrite H0; try rewrite H. rewrite <- nth_extend_left.
  replace (length l'') with (length l'' + 0) by auto with arith. rewrite <- nth_extend_left.
  replace (length l') with (length l' + 0) by auto with arith.
  now rewrite <- nth_extend_left.

  clear H0. now rewrite !nth_app_l by trivial.
  clear H0. rewrite !nth_app_r by lia. f_equal. lia.
Qed.

#[local] Hint Rewrite <- app_assoc in_app_iff in_inv : list.

Lemma type_lift Γ t T Γ' : Γ' @ Γ |-- t : T
   Γ'', Γ' @ Γ'' @ Γ |-- lift (length Γ') (length Γ'') t : T.
Proof.
  intros H.
  depind H; intros; simp lift; eauto with term.

  generalize (nth_extend_middle unit i Γ0 Γ' Γ'').
  destruct Nat.compare; intros H'; rewrite H'; simp lift;
    apply axiom; autorewrite with list in H |- *; lia.

  apply abstraction. rewrite app_comm_cons. now apply IHtypes.
Qed.

Lemma type_lift1 Γ t T A : Γ |-- t : T A :: Γ |-- lift 0 1 t : T.
Proof. intros. apply (type_lift Γ t T [] H [A]). Qed.

Lemma type_liftn Γ Γ' t T : Γ |-- t : T Γ' @ Γ |-- lift 0 (length Γ') t : T.
Proof. intros. apply (type_lift Γ t T [] H Γ'). Qed.
#[local] Hint Resolve type_lift1 type_lift type_liftn : term.

Ltac crush ::= do_rewrites; simpl; do_rewrites; auto; try term.

Lemma app_cons_snoc_app {A} l (a : A) l' : l ++ (a :: l') = (l ++ a :: nil) ++ l'.
Proof. induction l; crush. Qed.

#[local] Hint Extern 5 ⇒ progress (simpl ; autorewrite with list) : term.
Ltac term ::= simp lift subst; try typeclasses eauto with core term.

Lemma substitutive Γ t T Γ' u U :
  (Γ' @ (U :: Γ)) |-- t : T Γ |-- u : U
  Γ' @ Γ |-- subst (length Γ') t u : T.
Proof with term.
  intros H. depind H; term. intros.

  (* Var *)
  assert (spec:=Nat.compare_spec i (length Γ')).
  depelim spec; try fold (length Γ') in H1; subst;
  try rewrite H1; try rewrite H2 ; simp subst.

  (* Eq *)
  generalize (type_lift Γ0 u U [] H0 Γ'); simpl; intros.
  rewrite app_cons_snoc_app, app_nth1, app_nth2; try (simpl; lia).
  now rewrite <- minus_n_n. term.

  (* Lt *)
  rewrite app_nth1 by lia. rewrite <- (app_nth1 _ Γ0); term.

  (* Gt *)
  rewrite app_nth2; term.
  change (U :: Γ0) with ((cons U nil) @ Γ0). rewrite app_nth2; term.
  simpl. rewrite (nth_extend_left unit _ Γ0 Γ').
  replace (length Γ' + (i - length Γ' - 1)) with (pred i); term.
  apply axiom. autorewrite with list in H |- ×. simpl in H. lia.

  (* Abstraction *)
  intros. apply abstraction. now eapply (IHtypes _ _ _ (A :: Γ')).
Qed.

Lemma subst1 Γ t T u U : U :: Γ |-- t : T Γ |-- u : U Γ |-- subst 0 t u : T.
Proof. intros; now apply (substitutive Γ t T [] u U). Qed.

Reserved Notation " t --> u " (at level 55, right associativity).

Inductive reduce : term term Prop :=
| red_beta t u : @((Lambda t) , u) --> subst 0 t u
| red_fst t u : Fst << t , u >> --> t
| red_snd t u : Snd << t , u >> --> u

where " t --> u " := (reduce t u).
Derive Signature for reduce.

Require Import Relations.

Definition reduces := clos_refl_trans term reduce.
Notation " t -->* u " := (reduces t u) (at level 55).

Require Import Setoid.

Instance: Transitive reduces.
Proof. red; intros. econstructor 3; eauto. Qed.

Instance: Reflexive reduces.
Proof. red; intros. econstructor 2; eauto. Qed.

Inductive value : term Prop :=
| val_var (i : nat) : value i
| val_unit : value Tt
| val_pair a b : value a value b value << a, b >>
| val_lambda t : value λ t.
Derive Signature for value.

#[local] Hint Constructors value : term.

Inductive reduce_congr : relation term :=
| reduce1 t u : reduce t u reduce_congr t u
| reduce_app_l t t' u : reduce_congr t t'
  reduce_congr (@(t, u)) (@(t', u))
| reduce_app_r t u u' : reduce_congr u u'
  reduce_congr (@(t, u)) (@(t, u'))
| reduce_pair_l t t' u : reduce_congr t t'
  reduce_congr (<< t, u >>) (<< t', u >>)
| reduce_pair_r t u u' : reduce_congr u u'
  reduce_congr (<< t, u >>) (<< t, u' >>)
| reduce_fst t t' : reduce_congr t t' reduce_congr (Fst t) (Fst t')
| reduce_snd t t' : reduce_congr t t' reduce_congr (Snd t) (Snd t').
Derive Signature for reduce_congr.

Ltac find_empty :=
  match goal with
      [ H : _ |- _ ] ⇒ solve [ depelim H ]
  end.

Lemma preserves_red1 Γ t τ : Γ |-- t : τ u, t --> u Γ |-- u : τ.
Proof.
  intros H; induction H; intros t' redtt'; term; try find_empty; depelim redtt'.
  apply subst1 with A. now depelim H. apply H0.
  now depelim H.
  now depelim H.
Qed.

Lemma preserves_redpar Γ t τ : Γ |-- t : τ u, reduce_congr t u Γ |-- u : τ.
Proof.
  intros H. induction H; intros t' rtt'; depelim rtt'; term; try find_empty.

  depelim H1. depelim H. eapply subst1; eauto.

  depelim H0; depelim H; term.
  depelim H0; depelim H; term.
Qed.

Lemma subject_reduction Γ t τ : Γ |-- t : τ u, t -->* u Γ |-- u : τ.
Proof. induction 2; eauto using preserves_red1. Qed.
#[local] Hint Constructors reduce reduce_congr : term.
Lemma progress_ t τ : nil |-- t : τ ( t', reduce_congr t t') value t.
Proof.
  intros H; depind H; auto with term.

  destruct IHtypes1 as [[t' tt']|vt].
  left; eauto with term.
  destruct IHtypes2 as [[u' uu']|vu].
  left; eauto with term.
  depelim H; [depelim H|depelim vt..].
  left. ([u]t0). eauto with term.

  destruct IHtypes1 as [[t' tt']|vt]; eauto with term.
  destruct IHtypes2 as [[u' uu']|vu]; eauto with term.

  destruct IHtypes as [[t' tt']|vt]; eauto with term.
  depelim vt; depelim H;
  eauto with term. depelim H.

  destruct IHtypes as [[t' tt']|vt]; eauto with term.
  depelim vt; depelim H;
  eauto with term. depelim H.
Qed.

Reserved Notation " Γ |-- t => A " (at level 70, t, A at next level).
Reserved Notation " Γ |-- t <= A " (at level 70, t, A at next level).

Inductive atomic : type Prop :=
| atomic_atom a : atomic (atom a).

Derive Signature for atomic.
#[local] Hint Constructors atomic : term.

(* FIXME bug *)
Equations? atomic_dec (t : type) : { atomic t } + { ¬ atomic t } :=
atomic_dec (atom a) := left (atomic_atom a) ;
atomic_dec t := right _.
Proof. all:(intro H; depelim H). Qed.

Inductive check : ctx term type Prop :=
| abstraction_check Γ A B t :
  A :: Γ |-- t B
  Γ |-- λ t A ---> B

| pair_intro_check Γ A B t u :
  Γ |-- t A Γ |-- u B
    Γ |-- << t , u >> (A × B)

| unit_intro_check Γ : Γ |-- Tt unit

| check_synth Γ t T : atomic T Γ |-- t T Γ |-- t T

with synthetize : ctx term type Prop :=

| axiom_synth Γ i : i < length Γ
  Γ |-- i nth i Γ unit
 
| application_synth {Γ A B t u} :
  Γ |-- t A ---> B Γ |-- u A Γ |-- @(t, u) B

| pair_elim_fst_synth {Γ A B t} : Γ |-- t (A × B) Γ |-- Fst t A

| pair_elim_snd_synth {Γ A B t} : Γ |-- t (A × B) Γ |-- Snd t B

where "Γ |-- i => A " := (synthetize Γ i A)
and "Γ |-- i <= A " := (check Γ i A).
Derive Signature for check synthetize.

#[local] Hint Constructors synthetize check : term.

Scheme check_mut_ind := Elimination for check Sort Prop
  with synthetize_mut_ind := Elimination for synthetize Sort Prop.

Combined Scheme check_synthetize from check_mut_ind, synthetize_mut_ind.

Lemma synth_arrow {Γ t T} : A : Prop, Γ |-- λ (t) T A.
Proof. intros A H. depelim H. Qed.

Lemma synth_pair {Γ t u T} : A : Prop, Γ |-- << t, u >> T A.
Proof. intros A H. depelim H. Qed.

Lemma synth_unit {Γ T} : A : Prop, Γ |-- Tt T A.
Proof. intros A H. depelim H. Qed.

#[local] Hint Extern 3 ⇒
  match goal with
    | H : ?Γ |-- ?t ?T |- _apply (synth_arrow _ H) || apply (synth_pair _ H) || apply (synth_unit _ H)
  end : term.

Lemma check_types : ( Γ t T, Γ |-- t T Γ |-- t : T)
with synthetizes_types : ( Γ t T, Γ |-- t T Γ |-- t : T).
Proof. intros. destruct H; try econstructor; term.
  intros. destruct H; try solve [ econstructor; term ].
Qed.

#[local] Hint Resolve check_types synthetizes_types : term.

Inductive normal : term Prop :=
| normal_unit : normal Tt
| normal_pair a b : normal a normal b normal << a, b >>
| normal_abs t : normal t normal λ t
| normal_neutral r : neutral r normal r

with neutral : term Prop :=
| neutral_var i : neutral (Var i)
| neutral_fst t : neutral t neutral (Fst t)
| neutral_snd t : neutral t neutral (Snd t)
| neutral_app t n : neutral t normal n neutral (@(t, n)).

Derive Signature for normal neutral.
#[local] Hint Constructors normal neutral : term.

Lemma check_lift_gen Δ t T (H : Δ |-- t T) : Γ Γ', Δ = Γ' @ Γ
   Γ'', Γ' @ Γ'' @ Γ |-- lift (length Γ') (length Γ'') t T
with synthetize_lift_gen Δ t T (H : Δ |-- t T) : Γ Γ', Δ = Γ' @ Γ
   Γ'', Γ' @ Γ'' @ Γ |-- lift (length Γ') (length Γ'') t T.
Proof.
  destruct H; intros; simp lift.

  econstructor.
  change (S (length Γ')) with (length (A :: Γ')). change (A :: Γ' @ Γ'' @ Γ0) with ((A :: Γ') @ Γ'' @ Γ0).
  eapply check_lift_gen; try eassumption. subst. rewrite app_comm_cons; subst; try eassumption; trivial.

  econstructor; eapply check_lift_gen; eassumption.
  econstructor.

  econstructor. eassumption.
  eapply synthetize_lift_gen; eassumption.

  destruct H; intros; simp lift; try solve [econstructor; term].
  clear check_lift_gen synthetize_lift_gen. subst.
  generalize (nth_extend_middle unit i Γ0 Γ' Γ'').
  destruct Nat.compare; intros H'; rewrite H'; simp lift; apply axiom_synth; autorewrite with list in H |- *; lia.
Qed.

Definition check_lift Γ t T Γ' (H : Γ' @ Γ |-- t T) :
   Γ'', Γ' @ Γ'' @ Γ |-- lift (length Γ') (length Γ'') t T :=
  check_lift_gen (Γ' @ Γ) _ _ H _ _ eq_refl.
Definition synthetize_lift Γ t T Γ' (H : Γ' @ Γ |-- t T) :
   Γ'', Γ' @ Γ'' @ Γ |-- lift (length Γ') (length Γ'') t T :=
  synthetize_lift_gen (Γ' @ Γ) _ _ H _ _ eq_refl.

Lemma check_lift1 {Γ t T A} : Γ |-- t T A :: Γ |-- lift 0 1 t T.
Proof. intros. apply (check_lift Γ t T [] H [A]). Qed.

Lemma synth_lift1 {Γ t T A} : Γ |-- t T A :: Γ |-- lift 0 1 t T.
Proof. intros. apply (synthetize_lift Γ t T [] H [A]). Qed.
#[local] Hint Resolve check_lift1 synth_lift1 : term.

Lemma check_lift_ctx {Γ t T Γ'} : Γ |-- t T Γ' @ Γ |-- lift 0 (length Γ') t T.
Proof. intros. apply (check_lift Γ t T [] H Γ'). Qed.

Lemma synth_lift_ctx {Γ t T Γ'} : Γ |-- t T Γ' @ Γ |-- lift 0 (length Γ') t T.
Proof. intros. apply (synthetize_lift Γ t T [] H Γ'). Qed.
#[local] Hint Resolve check_lift_ctx synth_lift_ctx : term.

Equations η (a : type) (t : term) : term :=
η (atom _) t := t ;
η (product a b) t := << η a (Fst t), η b (Snd t) >> ;
η (arrow a b) t := (Lambda (η b @(lift 0 1 t, η a 0)))%term ;
η unit t := Tt.

Lemma checks_arrow Γ t A B : Γ |-- t A ---> B t', t = λ t' A :: Γ |-- t' B.
Proof. intros H; inversion H; subst.
   t0; term.
  inversion H0.
Qed.

Lemma normal_lift {t k n} : normal t normal (lift k n t)
  with neutral_lift {t k n} : neutral t neutral (lift k n t).
Proof. destruct 1; simp lift; constructor; term.
  destruct 1; simp lift; try (constructor; term).
  destruct Nat.compare; term.
Qed.
#[local] Hint Resolve normal_lift neutral_lift : term.

Lemma check_normal {Γ t T} : Γ |-- t T normal t
 with synth_neutral {Γ t T} : Γ |-- t T neutral t.
Proof. destruct 1; constructor; term. destruct 1; constructor; term. Qed.
#[local] Hint Resolve check_normal synth_neutral : term.

Lemma eta_expand Γ t A : neutral t Γ |-- t A Γ |-- η A t A.
Proof. revert Γ t; induction A; intros; simp η; constructor; term.

  assert(0 < length (A1 :: Γ)) by (simpl; lia).
  specialize (IHA1 (A1 :: Γ) 0 (neutral_var _) (axiom_synth (A1 :: Γ) 0 H1)).
  apply (IHA2 (A1 :: Γ) @(lift 0 1 t, η A1 0)); term.
Qed.

Lemma η_normal : Γ A t, neutral t Γ |-- t A normal (η A t).
Proof. intros. now apply eta_expand in H0; term. Qed.

Going to use the subterm order

Require Import Arith Wf_nat.
Instance wf_nat : Classes.WellFounded lt := lt_wf.

#[local] Hint Constructors Subterm.lexprod : subterm_relation.

Derive Signature for Acc.
Notation lexicographic R S := (Subterm.lexprod _ _ R S).

Definition her_order : relation (type × term × term) :=
  lexicographic (lexicographic type_subterm term_subterm) term_subterm.

#[local] Hint Unfold her_order : subterm_relation.

Import Program.Tactics.
Obligation Tactic := program_simpl.

Arguments exist [A] [P].

Definition hereditary_type (t : type × term × term) :=
  (term × option { u : type | u = (fst (fst t)) type_subterm u (fst (fst t)) })%type.

Inductive IsLambda {t} : hereditary_type t Set :=
| isLambda abs a b prf : IsLambda (Lambda abs, Some (exist (arrow a b) prf)).

Equations is_lambda {t} (h : hereditary_type t) : IsLambda h + term :=
is_lambda (pair (Lambda abs) (Some (exist (arrow a b) prf))) := inl (isLambda abs a b prf) ;
is_lambda (pair t' _) := inr t'.
Arguments is_lambda : simpl never.
Lemma is_lambda_inr {t} (h : hereditary_type t) : t', is_lambda h = inr t' fst h = t'.
Proof.
  let elim := constr:(fun_elim (f:=@is_lambda)) in apply elim; simpl; intros; try congruence.
Qed.

Inductive IsPair {t} : hereditary_type t Set :=
| isPair u v a b prf : IsPair (Pair u v, Some (exist (product a b) prf)).

Equations is_pair {t} (h : hereditary_type t) : IsPair h + term :=
is_pair (pair (Pair u v) (Some (exist (product a b) prf))) := inl (isPair u v a b prf) ;
is_pair (pair t' _) := inr t'.
Arguments is_pair : simpl never.

Lemma is_pair_inr {t} (h : hereditary_type t) : t', is_pair h = inr t' fst h = t'.
Proof.
  let elim := constr:(fun_elim (f:=@is_pair)) in apply elim; simpl; intros; try congruence.
Qed.

Lemma nth_extend_right {A} (a : A) n (l l' : list A) : n < length l
  nth n l a = nth n (l @ l') a.
Proof. revert n l'. induction l; simpl; intros; auto. depelim H. destruct n; auto.
  apply IHl. auto with arith.
Qed.

Definition her_type (t : type × term × term) :=
  let u' := fst (fst t) in
   { u : type | u = u' type_subterm u u' }.

#[local] Remove Hints t_step : subterm_relation.
#[local] Remove Hints Subterm.clos_trans_stepr : subterm_relation.

Ltac apply_step :=
  match goal with
    |- clos_trans ?A ?R ?x ?ynot_evar y; eapply t_step
  end.
#[local] Hint Extern 30 (clos_trans _ _ _ _) ⇒ apply_step : subterm_relation.

Lemma clos_trans_inv {A} R (x y z : A) :
  clos_trans A R y z clos_trans A R x y clos_trans A R x z.
Proof. eauto using t_trans. Qed.

Ltac apply_transitivity :=
  match goal with
    |- clos_trans ?A ?R ?x ?y
    not_evar x; not_evar y; eapply clos_trans_inv
  end.
#[local] Hint Extern 31 (clos_trans _ _ _ _) ⇒ apply_transitivity : subterm_relation.

Equations? hereditary_subst (t : type × term × term) (k : nat) :
  term × option (her_type t)
  by wf t her_order :=

hereditary_subst (pair (pair A a) t) k with t := {
  | Var i with Nat.compare i k := {
    | Eq := (lift 0 k a, Some (exist A _)) ;
    | Lt := (Var i, None) ;
    | Gt := (Var (pred i), None) } ;

  | Lambda t' := (Lambda (fst (hereditary_subst (A, a, t') (S k))), None) ;

  | App f arg with hereditary_subst (A, a, f) k := {
    | p with is_lambda p := {
        | inl (isLambda f' A' B' prf) :=
          let (f'', y) := hereditary_subst (A', fst (hereditary_subst (A, a, arg) k), f') 0 in
            (f'', Some (exist B' _)) ;
        | inr f' := (@(f', fst (hereditary_subst (A, a, arg) k)), None) } } ;

  | Pair i j :=
    (<< fst (hereditary_subst (A, a, i) k), fst (hereditary_subst (A, a, j) k) >>, None) ;

  | Fst t' with hereditary_subst (A, a, t') k := {
    | p with is_pair p := {
      | inl (isPair u v a' b' prf) := (u, Some (exist a' _)) ;
      | inr p' := (Fst p', None) } } ;

  | Snd t' with hereditary_subst (A, a, t') k := {
    | p with is_pair p := {
      | inl (isPair u v a' b' prf) := (v, Some (exist b' _)) ;
      | inr p' := (Snd p', None) } } ;

  | Tt := (Tt, None) }.
Proof.
  all:(try match goal with |- her_order _ _
                       unfold her_type in *; simpl in *; try (clear; constructor 2; do 2 constructor)
       end).
  1:(destruct prf; subst; eauto 10 with subterm_relation).
  all:(clear -prf; simpl in *; destruct prf; subst; eauto 5 with subterm_relation).
Defined.

#[local] Hint Unfold her_type : subterm_relation.
#[local] Hint Unfold Program.Basics.const : subterm_relation.

Ltac autoh :=
  unfold type_subterm in × ; try typeclasses eauto with hereditary_subst subterm_relation.
Ltac simph :=
  try (rewrite_strat (innermost (hints hereditary_subst)));
  autoh.

#[local] Hint Transparent type_subterm : subterm_relation.

Ltac invert_term :=
  match goal with
    | [ H : check _ (Lambda _) _ |- _ ] ⇒ depelim H
    | [ H : check _ (Pair _ _) _ |- _ ] ⇒ depelim H
    | [ H : check _ Tt _ |- _ ] ⇒ depelim H
    | [ H : types _ ?t _ |- _ ] ⇒
      match t with
        | Var _depelim H
        | Lambda _depelim H
        | App _ _depelim H
        | Pair _ _depelim H
        | Fst _depelim H
        | Snd _depelim H
        | Ttdepelim H
      end
  end.

Set Regular Subst Tactic.

Lemma hereditary_subst_type Γ Γ' t T u U : Γ |-- u : U Γ' @ (U :: Γ) |-- t : T
  let (t', o) := hereditary_subst (U, u, t) (length Γ') in
    (Γ' @ Γ |-- t' : T ( ty prf, o = Some (exist ty prf) ty = T)).
Proof.
  intros.
  funelim (hereditary_subst (U, u, t) (length Γ'));
    DepElim.simpl_dep_elim; subst;
    try (split; [ (intros; try discriminate) | solve [ intros; discriminate ] ]);
    DepElim.simplify_dep_elim.

  invert_term. simpl in ×. apply abstraction.
  specialize (H Γ (A :: Γ')). simpl in H. eqns_specialize_eqs H.
  simpl in H.
  on_call hereditary_subst ltac:(fun cremember c as hsubst; destruct hsubst; simpl in *).
  specialize (H _ H0 H1).
  apply H; auto.

  on_call hereditary_subst ltac:(fun cremember c as hsubst; destruct hsubst; simpl in *).
  on_call hereditary_subst ltac:(fun cremember c as hsubst; destruct hsubst; simpl in *).
  depelim H2. constructor. now apply H. now apply H0.
  depelim H0. term.

  (* Var *) simpl.
  apply Nat.compare_eq in Heq; subst.
  depelim H0.
  rewrite !nth_length. split. term. intros.
  noconf H1. auto.

  (* Lt *)
  apply Nat.compare_lt_iff in Heq. depelim H0.
  replace (nth n (Γ' @ (_ :: Γ)) unit) with (nth n (Γ' @ Γ) unit).
  constructor. rewrite app_length. auto with arith.
  now do 2 rewrite <- nth_extend_right by auto.

  (* Gt *)
  pose (substitutive _ _ _ _ _ _ H0 H).
  simp subst in t. rewrite Heq in t. simp subst in t.

  (* App *)
  simpl in ×.
  on_call (hereditary_subst (t0, t1, u)) ltac:(fun cremember c as hsubst; destruct hsubst; simpl in *).
  on_call hereditary_subst ltac:(fun cremember c as hsubst; destruct hsubst; simpl in × ).
  dependent elimination H2 as [application _ T U fn arg tyfn tyu].
  specialize (H _ _ H1 tyu).
  specialize (Hind _ _ H1 tyfn).
  rewrite Heq in Hind. destruct Hind as [Ht' Ht''].
  dependent elimination Ht' as [abstraction _ T U abs tyabs].
  eqns_specialize_eqs Ht''. noconf Ht''.
  destruct H as [Ht tty].
  specialize (H0 _ [] _ _ _ _ Ht tyabs eq_refl).
  destruct H0 as [H0 H5].
  split; auto.
  intros ty prf0 Heq'.
  noconf Heq'. auto.

  (* App no redex *)
  apply is_lambda_inr in Heq. revert Heq.
  on_call hereditary_subst ltac:(fun cremember c as hsubst; destruct hsubst; simpl in × ).
  intros. subst t2. depelim H1.
  apply application with A; eauto.
  eapply Hind; eauto.
  destruct_call hereditary_subst.
  eapply H; eauto.

  (* Fst redex *)
  simpl in ×.
  depelim H0. specialize (Hind _ _ H H0).
  rewrite Heq in Hind.
  destruct Hind. depelim H1. intuition auto.
  eqns_specialize_eqs H2. noconf H2.
  now noconf H1.

  (* Fst no redex *)
  apply is_pair_inr in Heq. revert Heq.
  on_call hereditary_subst ltac:(fun cremember c as hsubst; destruct hsubst; simpl in × ).
  depelim H0. intros <-.
  specialize (Hind _ _ H H0); eauto.
  destruct Hind. now apply pair_elim_fst with B.

  (* Snd redex *)
  simpl. depelim H0. specialize (Hind _ _ H H0).
  rewrite Heq in Hind.
  destruct Hind. depelim H1. intuition auto.
  eqns_specialize_eqs H2. noconf H2.
  now noconf H1.

  (* Snd no redex *)
  apply is_pair_inr in Heq. revert Heq.
  on_call hereditary_subst ltac:(fun cremember c as hsubst; destruct hsubst; simpl in × ).
  intros Ht2; subst t2. simpl in ×. depelim H0.
  specialize (Hind _ _ H H0); eauto. now apply pair_elim_snd with A.
Qed.
Print Assumptions hereditary_subst_type.
Import Program.Basics.
Instance: subrelation eq (flip impl).
Proof. reduce. subst; auto. Qed.

Lemma nth_pred Γ' Γ U n : n > length Γ' nth (pred n) (Γ' @ Γ) unit = nth n (Γ' @ (U :: Γ)) unit.
Proof.
  revert_until Γ'. induction Γ'; intros.

  destruct n; auto. depelim H.
  destruct n; auto. simpl pred. simpl.
  rewrite <- IHΓ'. destruct n; auto. simpl in H. depelim H. depelim H.
  simpl in *; lia.
Qed.

Lemma hereditary_subst_subst U u t Γ' :
  ( Γ T, Γ |-- u U
    match hereditary_subst (U, u, t) (length Γ') with
      | (t', Some (exist ty _))
         ((Γ' @ (U :: Γ) |-- t T Γ' @ Γ |-- t' T ty = T)
          (Γ' @ (U :: Γ) |-- t T Γ' @ Γ |-- t' T ty = T))
      | (t', None)
        (Γ' @ (U :: Γ) |-- t T Γ' @ Γ |-- t' T)
        (Γ' @ (U :: Γ) |-- t T Γ' @ Γ |-- t' T)
    end).
Proof.
  funelim (hereditary_subst (U, u, t) (length Γ')); simpl in ×.
  let Hind := fresh "Hind" in rename H into Hind; intros ?? Hu.
  simpl. simpl in ×.

Lambda
  - on_call hereditary_subst
            ltac:(fun cremember c as hsubst; destruct hsubst; simpl in *).
    split; intros Hsyn; [| elim (synth_arrow False Hsyn)].

    invert_term. constructor.
    specialize (Hind t0 _ _ (A :: Γ') eq_refl). simpl in ×.
    specialize (Hind _ B Hu).
    destruct o as [[ty prf]|], Hind as [Hind0 Hind1].
    apply Hind0; eauto. eauto.
    elim (synth_arrow False H0).

Pairs
  - do 2 on_call hereditary_subst
          ltac:(fun cremember c as hsubst; destruct hsubst; simpl in *).
    split; intros Hsyn; [|elim (synth_pair False Hsyn)].
    invert_term.
    specialize (H0 _ B H1). specialize (H _ A H1).
    destruct o as [[ty prf]|], o0 as [[ty' prf']|], H, H0;
      destruct_conjs; constructor; eauto.
    now apply H. now apply H0. now apply H. now apply H0.

    elim (synth_pair False H3).

  (* Unit *)
  - split; intros Hsyn; [|elim (synth_unit False Hsyn)].
    depelim Hsyn. term.
    elim (synth_unit False H1).

  (* Var: eq *)
  - apply Nat.compare_eq in Heq; subst n.
    split; intros Hsyn; depelim Hsyn; rewrite ?nth_length.
    depelim H1; rewrite !nth_length.
    now split; term. split; term.

  (* Lt *)
  - apply Nat.compare_lt_iff in Heq.
    split; intros Hsyn; depelim Hsyn;
    [depelim H1;constructor;auto|];
    (rewrite nth_app_l by lia; rewrite <- nth_app_l with (l':=Γ) by lia;
     constructor; rewrite app_length; auto with arith).

  (* Gt *)
  - apply Nat.compare_gt_iff in Heq.
    split; intros Hsyn; depelim Hsyn.
    depelim H1. constructor. auto.
    replace (nth n (Γ' @ (t0 :: Γ)) unit) with (nth (pred n) (Γ' @ Γ) unit).
    constructor. rewrite app_length in ×. simpl in H1. lia.
    now apply nth_pred.

    replace (nth _ (Γ' @ (_ :: _)) unit) with (nth (pred n) (Γ' @ Γ) unit).
    constructor. rewrite app_length in ×. simpl in H0. lia.
    now apply nth_pred.

  (* App *)
  - on_call (hereditary_subst (t0,t1,u))
            ltac:(fun cremember c as hsubst; destruct hsubst; simpl in *).
    specialize (H0 _ _ _ [] eq_refl).
    rewrite Heq in Hind.
    revert H0.
    on_call hereditary_subst
            ltac:(fun cremember c as hsubst; destruct hsubst; simpl in *).
    intros.

    (* Redex *)
    assert((Γ' @ (t0 :: Γ) |-- @( t3, u) T Γ' @ Γ |-- t2 T b = T)).
    intros Ht; depelim Ht.
    destruct (Hind Γ (A ---> T) H1).
    specialize (H _ A H1).
    destruct (H4 Ht). noconf H6.
    depelim H5. split; auto.

    destruct o; try destruct h; destruct H.
    destruct (H H2). subst x.
    specialize (H0 _ b H7).
    destruct o0 as [[ty typrf]|]; destruct H0 as [Hcheck Hinf].
    now apply Hcheck. now apply Hcheck.

    specialize (H0 _ b (H H2)).
    destruct o0 as [[ty typrf]|]; destruct H0 as [Hcheck Hinf].
    now apply Hcheck. now apply Hcheck.

    split; auto.
    depelim H6.

    split; eauto.
    intros Ht3u; apply H2.
    now depelim Ht3u.

  (* No redex *)
  - intros Γ T Hu.
    assert(Γ' @ (t0 :: Γ) |-- @( t3, u) T
       Γ' @ Γ |-- @( t2, fst (hereditary_subst (t0, t1, u) (length Γ'))) T).
    intros Ht; depelim Ht.
    on_call hereditary_subst
            ltac:(fun cremember c as hsubst; destruct hsubst; simpl in *).
    revert Heq.
    on_call hereditary_subst
            ltac:(fun cremember c as hsubst; destruct hsubst; simpl in *).
    intros.
    pose (Hind _ (A ---> T) Hu).
    destruct o0 as [[ty prf']|].
    + destruct y as [Hind' Hind''].
      specialize (Hind'' Ht). destruct Hind''; subst ty.
      specialize (H _ A Hu).
      destruct o as [[ty' prf'']|].
      ++ destruct H as [Hind0 Hind0'].
        specialize (Hind0 H0). destruct Hind0. subst ty'.
        eapply application_synth; eauto. simpl in ×.
        depelim H1. simp is_lambda in Heq. noconf Heq.
        depelim H1.

      ++ depelim H1. simp is_lambda in Heq. noconf Heq. depelim H1.
    + clear y. specialize (H _ A Hu).
      destruct (Hind _ (A ---> T) Hu).
      apply is_lambda_inr in Heq. subst t2. simpl.
      destruct o as [[ty prf]|]; destruct H as [Hindt0 Hindt0'].
      eapply application_synth; eauto.
      now apply Hindt0.
      eapply application_synth; eauto.

    + split; auto. intros H2.
      depelim H2.
      constructor; auto.

  (* Pair *)
  - simpl in Heq. autorewrite with is_pair in Heq. simpl in prf.
    intros Γ T Hu.
    assert( (Γ' @ (t0 :: Γ) |-- Fst t5 T Γ' @ Γ |-- u T a = T)).
    intros Ht; depelim Ht. specialize (Hind _ (T × B) Hu). revert Hind.
    on_call hereditary_subst ltac:(fun cremember c as hsubst; destruct hsubst; simpl in *).
    noconf Heq.
    intros [Hind Hind'].
    specialize (Hind' Ht). destruct Hind' as [H0 H1]. noconf H1.
    depelim H0. split; auto.
    depelim H0.
    split; auto.
    intros H1. depelim H1. intuition.

  - intros Γ T Hu.
    assert (Γ' @ (t0 :: Γ) |-- Fst t5 T Γ' @ Γ |-- Fst t2 T).
    intros Ht; depelim Ht.
    specialize (Hind _ (T × B) Hu). revert Hind.
    on_call hereditary_subst ltac:(fun cremember c as hsubst; destruct hsubst; simpl in *).
    destruct o as [[ty prf]|]. intros [Hind Hind'].
    destruct (Hind' Ht). subst ty.
    depelim H. simp is_pair in Heq. discriminate.
    depelim H.

    apply is_pair_inr in Heq. simpl in Heq ; subst t2.
    intros [Hind Hind']. eapply pair_elim_fst_synth. now apply Hind'.
    split; auto. intros H2. depelim H2. intuition auto with term.

  (* Snd *)
  - intros Γ T Hu.
    assert((Γ' @ (t0 :: Γ) |-- Snd t6 T Γ' @ Γ |-- v T b = T)).

    intros Ht; depelim Ht. specialize (Hind _ (A × T) Hu). revert Hind.
    on_call hereditary_subst
            ltac:(fun cremember c as hsubst; destruct hsubst; simpl in *).
    noconf Heq.
    intros [Hind Hind'].
    specialize (Hind' Ht). destruct Hind' as [H0 H1]. noconf H1.
    depelim H0. split; auto. depelim H0.
    split; auto.
    intros H1. depelim H1. intuition auto with term.

  - intros Γ T Hu.
    assert (Γ' @ (t0 :: Γ) |-- Snd t6 T Γ' @ Γ |-- Snd t2 T).
    intros Ht; depelim Ht.
    specialize (Hind _ (A × T) Hu). revert Hind.
    on_call hereditary_subst
            ltac:(fun cremember c as hsubst; destruct hsubst; simpl in *).
    destruct o as [[ty prf]|]. intros [Hind Hind'].
    destruct (Hind' Ht). subst ty.
    depelim H. simp is_pair in Heq. discriminate.
    depelim H.

    intros [Hind Hind'].
    apply is_pair_inr in Heq. subst t2. simpl in ×.
    specialize (Hind' Ht). econstructor; eauto.

    split; auto. intros H1. depelim H1. term.
Qed.

Print Assumptions hereditary_subst_subst.

Lemma check_liftn {Γ Γ' t T} : Γ |-- t T Γ' @ Γ |-- lift 0 (length Γ') t T.
Proof. intros. apply (check_lift Γ t T [] H Γ'). Qed.

Lemma synth_liftn {Γ Γ' t T} : Γ |-- t T Γ' @ Γ |-- lift 0 (length Γ') t T.
Proof. intros. apply (synthetize_lift Γ t T [] H Γ'). Qed.
#[local] Hint Resolve check_liftn synth_liftn : term.

(* Write normalization function *)
Lemma types_normalizes Γ t T : Γ |-- t : T u, Γ |-- u T.
Proof. induction 1. (* eta-exp *)

   (η (nth i Γ unit) i).
  apply (eta_expand Γ i (nth i Γ unit) (neutral_var _)); term.

  destruct IHtypes as [t' tt'].
   λ t'; term.

  destruct IHtypes1 as [t' tt'].
  destruct IHtypes2 as [u' uu'].

  (* Hereditary substitution *)
  apply checks_arrow in tt'. destruct tt' as [t'' [t't'' t'B]]. subst.

  generalize (hereditary_subst_subst _ _ t'' [] Γ B uu').
  destruct_call hereditary_subst. destruct o. destruct h.
  simpl in ×. intros. destruct H1. t0; intuition.
  simpl in ×. intros. destruct H1. t0; intuition.

  (* Unit *)
   Tt; term.

  (* Pair *)
  destruct IHtypes1 as [t' tt'].
  destruct IHtypes2 as [u' uu'].
   << t' , u' >>. term.

  (* Fst *)
  destruct IHtypes as [t' tt'].
  depelim tt'. t0; term.

  depelim H0.

  (* Snd *)
  destruct IHtypes as [t' tt'].
  depelim tt'. u; term.

  depelim H0.
Qed.

Print Assumptions types_normalizes.

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