diff --git a/syntax.sig b/syntax.sig
index ebc117b..00ef8ec 100644
--- a/syntax.sig
+++ b/syntax.sig
@@ -1,3 +1,4 @@
+nat  : Type
 Tm(VarTm) : Type
 PTag : Type
 
@@ -8,4 +9,5 @@ App : Tm -> Tm -> Tm
 Pair : Tm -> Tm -> Tm
 Proj : PTag -> Tm -> Tm
 Pi : Tm -> (bind Tm in Tm) -> Tm
-Bot : Tm
\ No newline at end of file
+Bot : Tm
+Univ : nat -> Tm
\ No newline at end of file
diff --git a/theories/Autosubst2/syntax.v b/theories/Autosubst2/syntax.v
index 3424c27..285bc2c 100644
--- a/theories/Autosubst2/syntax.v
+++ b/theories/Autosubst2/syntax.v
@@ -26,7 +26,8 @@ Inductive Tm (n_Tm : nat) : Type :=
   | Pair : Tm n_Tm -> Tm n_Tm -> Tm n_Tm
   | Proj : PTag -> Tm n_Tm -> Tm n_Tm
   | Pi : Tm n_Tm -> Tm (S n_Tm) -> Tm n_Tm
-  | Bot : Tm n_Tm.
+  | Bot : Tm n_Tm
+  | Univ : nat -> Tm n_Tm.
 
 Lemma congr_Abs {m_Tm : nat} {s0 : Tm (S m_Tm)} {t0 : Tm (S m_Tm)}
   (H0 : s0 = t0) : Abs m_Tm s0 = Abs m_Tm t0.
@@ -71,6 +72,12 @@ Proof.
 exact (eq_refl).
 Qed.
 
+Lemma congr_Univ {m_Tm : nat} {s0 : nat} {t0 : nat} (H0 : s0 = t0) :
+  Univ m_Tm s0 = Univ m_Tm t0.
+Proof.
+exact (eq_trans eq_refl (ap (fun x => Univ m_Tm x) H0)).
+Qed.
+
 Lemma upRen_Tm_Tm {m : nat} {n : nat} (xi : fin m -> fin n) :
   fin (S m) -> fin (S n).
 Proof.
@@ -93,6 +100,7 @@ Fixpoint ren_Tm {m_Tm : nat} {n_Tm : nat} (xi_Tm : fin m_Tm -> fin n_Tm)
   | Proj _ s0 s1 => Proj n_Tm s0 (ren_Tm xi_Tm s1)
   | Pi _ s0 s1 => Pi n_Tm (ren_Tm xi_Tm s0) (ren_Tm (upRen_Tm_Tm xi_Tm) s1)
   | Bot _ => Bot n_Tm
+  | Univ _ s0 => Univ n_Tm s0
   end.
 
 Lemma up_Tm_Tm {m : nat} {n_Tm : nat} (sigma : fin m -> Tm n_Tm) :
@@ -119,6 +127,7 @@ Fixpoint subst_Tm {m_Tm : nat} {n_Tm : nat} (sigma_Tm : fin m_Tm -> Tm n_Tm)
   | Pi _ s0 s1 =>
       Pi n_Tm (subst_Tm sigma_Tm s0) (subst_Tm (up_Tm_Tm sigma_Tm) s1)
   | Bot _ => Bot n_Tm
+  | Univ _ s0 => Univ n_Tm s0
   end.
 
 Lemma upId_Tm_Tm {m_Tm : nat} (sigma : fin m_Tm -> Tm m_Tm)
@@ -158,6 +167,7 @@ subst_Tm sigma_Tm s = s :=
       congr_Pi (idSubst_Tm sigma_Tm Eq_Tm s0)
         (idSubst_Tm (up_Tm_Tm sigma_Tm) (upId_Tm_Tm _ Eq_Tm) s1)
   | Bot _ => congr_Bot
+  | Univ _ s0 => congr_Univ (eq_refl s0)
   end.
 
 Lemma upExtRen_Tm_Tm {m : nat} {n : nat} (xi : fin m -> fin n)
@@ -201,6 +211,7 @@ Fixpoint extRen_Tm {m_Tm : nat} {n_Tm : nat} (xi_Tm : fin m_Tm -> fin n_Tm)
         (extRen_Tm (upRen_Tm_Tm xi_Tm) (upRen_Tm_Tm zeta_Tm)
            (upExtRen_Tm_Tm _ _ Eq_Tm) s1)
   | Bot _ => congr_Bot
+  | Univ _ s0 => congr_Univ (eq_refl s0)
   end.
 
 Lemma upExt_Tm_Tm {m : nat} {n_Tm : nat} (sigma : fin m -> Tm n_Tm)
@@ -245,6 +256,7 @@ Fixpoint ext_Tm {m_Tm : nat} {n_Tm : nat} (sigma_Tm : fin m_Tm -> Tm n_Tm)
         (ext_Tm (up_Tm_Tm sigma_Tm) (up_Tm_Tm tau_Tm) (upExt_Tm_Tm _ _ Eq_Tm)
            s1)
   | Bot _ => congr_Bot
+  | Univ _ s0 => congr_Univ (eq_refl s0)
   end.
 
 Lemma up_ren_ren_Tm_Tm {k : nat} {l : nat} {m : nat} (xi : fin k -> fin l)
@@ -289,6 +301,7 @@ Fixpoint compRenRen_Tm {k_Tm : nat} {l_Tm : nat} {m_Tm : nat}
         (compRenRen_Tm (upRen_Tm_Tm xi_Tm) (upRen_Tm_Tm zeta_Tm)
            (upRen_Tm_Tm rho_Tm) (up_ren_ren _ _ _ Eq_Tm) s1)
   | Bot _ => congr_Bot
+  | Univ _ s0 => congr_Univ (eq_refl s0)
   end.
 
 Lemma up_ren_subst_Tm_Tm {k : nat} {l : nat} {m_Tm : nat}
@@ -343,6 +356,7 @@ Fixpoint compRenSubst_Tm {k_Tm : nat} {l_Tm : nat} {m_Tm : nat}
         (compRenSubst_Tm (upRen_Tm_Tm xi_Tm) (up_Tm_Tm tau_Tm)
            (up_Tm_Tm theta_Tm) (up_ren_subst_Tm_Tm _ _ _ Eq_Tm) s1)
   | Bot _ => congr_Bot
+  | Univ _ s0 => congr_Univ (eq_refl s0)
   end.
 
 Lemma up_subst_ren_Tm_Tm {k : nat} {l_Tm : nat} {m_Tm : nat}
@@ -418,6 +432,7 @@ ren_Tm zeta_Tm (subst_Tm sigma_Tm s) = subst_Tm theta_Tm s :=
         (compSubstRen_Tm (up_Tm_Tm sigma_Tm) (upRen_Tm_Tm zeta_Tm)
            (up_Tm_Tm theta_Tm) (up_subst_ren_Tm_Tm _ _ _ Eq_Tm) s1)
   | Bot _ => congr_Bot
+  | Univ _ s0 => congr_Univ (eq_refl s0)
   end.
 
 Lemma up_subst_subst_Tm_Tm {k : nat} {l_Tm : nat} {m_Tm : nat}
@@ -494,6 +509,7 @@ subst_Tm tau_Tm (subst_Tm sigma_Tm s) = subst_Tm theta_Tm s :=
         (compSubstSubst_Tm (up_Tm_Tm sigma_Tm) (up_Tm_Tm tau_Tm)
            (up_Tm_Tm theta_Tm) (up_subst_subst_Tm_Tm _ _ _ Eq_Tm) s1)
   | Bot _ => congr_Bot
+  | Univ _ s0 => congr_Univ (eq_refl s0)
   end.
 
 Lemma renRen_Tm {k_Tm : nat} {l_Tm : nat} {m_Tm : nat}
@@ -609,6 +625,7 @@ Fixpoint rinst_inst_Tm {m_Tm : nat} {n_Tm : nat}
         (rinst_inst_Tm (upRen_Tm_Tm xi_Tm) (up_Tm_Tm sigma_Tm)
            (rinstInst_up_Tm_Tm _ _ Eq_Tm) s1)
   | Bot _ => congr_Bot
+  | Univ _ s0 => congr_Univ (eq_refl s0)
   end.
 
 Lemma rinstInst'_Tm {m_Tm : nat} {n_Tm : nat} (xi_Tm : fin m_Tm -> fin n_Tm)
@@ -807,6 +824,8 @@ Core.
 
 Arguments VarTm {n_Tm}.
 
+Arguments Univ {n_Tm}.
+
 Arguments Bot {n_Tm}.
 
 Arguments Pi {n_Tm}.
diff --git a/theories/fp_red.v b/theories/fp_red.v
index 902c733..a110d51 100644
--- a/theories/fp_red.v
+++ b/theories/fp_red.v
@@ -74,7 +74,9 @@ Module Par.
     R (Pi A0 B0) (Pi A1 B1)
   (* Bot is useful for making the prov function computable  *)
   | BotCong :
-    R Bot Bot.
+    R Bot Bot
+  | UnivCong i :
+    R (Univ i) (Univ i).
 
   Lemma refl n (a : Tm n) : R a a.
     elim : n /a; hauto ctrs:R.
@@ -135,6 +137,7 @@ Module Par.
     - qauto l:on ctrs:R.
     - hauto l:on inv:option ctrs:R use:renaming.
     - sfirstorder.
+    - sfirstorder.
   Qed.
 
   Lemma substing n m (a b : Tm n) (ρ : fin n -> Tm m) :
@@ -212,7 +215,8 @@ Module Par.
       move : ihB => [c0 [? ?]]. subst.
       eexists. split. by apply PiCong; eauto.
       by asimpl.
-    - hauto lq:on rew:off inv:Tm.
+    - move => n n0 ξ []//=. hauto l:on.
+    - move => n i n0 ξ []//=. hauto l:on.
   Qed.
 
 End Par.
@@ -287,7 +291,9 @@ Module RPar.
     R B0 B1 ->
     R (Pi A0 B0) (Pi A1 B1)
   | BotCong :
-    R Bot Bot.
+    R Bot Bot
+  | UnivCong i :
+    R (Univ i) (Univ i).
 
   Derive Dependent Inversion inv with (forall n (a b : Tm n), R a b) Sort Prop.
 
@@ -354,6 +360,7 @@ Module RPar.
     - hauto lq:on ctrs:R.
     - hauto lq:on ctrs:R use:morphing_up.
     - hauto lq:on ctrs:R.
+    - hauto lq:on ctrs:R.
   Qed.
 
   Lemma substing n m (a b : Tm n) (ρ : fin n -> Tm m) :
@@ -402,7 +409,9 @@ Module EPar.
     R B0 B1 ->
     R (Pi A0 B0) (Pi A1 B1)
   | BotCong :
-    R Bot Bot.
+    R Bot Bot
+  | UnivCong i :
+    R (Univ i) (Univ i).
 
   Lemma refl n (a : Tm n) : EPar.R a a.
   Proof.
@@ -446,6 +455,7 @@ Module EPar.
     - hauto q:on ctrs:R.
     - hauto l:on ctrs:R use:renaming inv:option.
     - hauto lq:on ctrs:R.
+    - hauto lq:on ctrs:R.
   Qed.
 
   Lemma substing n a0 a1 (b0 b1 : Tm n) :
@@ -746,6 +756,7 @@ Proof.
     + hauto lq:on ctrs:EPar.R use:RPars.ProjCong.
   - hauto lq:on inv:RPar.R ctrs:EPar.R, rtc use:RPars.PiCong.
   - hauto l:on ctrs:EPar.R inv:RPar.R.
+  - hauto l:on ctrs:EPar.R inv:RPar.R.
 Qed.
 
 Lemma commutativity1 n (a b0 b1 : Tm n) :
@@ -859,6 +870,20 @@ Lemma Bot_EPar' n (u : Tm n) :
   - hauto l:on ctrs:OExp.R.
 Qed.
 
+Lemma Univ_EPar' n i (u : Tm n) :
+  EPar.R (Univ i) u ->
+  rtc OExp.R (Univ i) u.
+  move E : (Univ i) => t h.
+  move : E. elim : n t u /h => //=.
+  - move => n a0 a1 h ih ?. subst.
+    specialize ih with (1 := eq_refl).
+    hauto lq:on ctrs:OExp.R use:rtc_r.
+  - move => n a0 a1 h ih ?. subst.
+    specialize ih with (1 := eq_refl).
+    hauto lq:on ctrs:OExp.R use:rtc_r.
+  - hauto l:on ctrs:OExp.R.
+Qed.
+
 Lemma EPar_diamond n (c a1 b1 : Tm n) :
   EPar.R c a1 ->
   EPar.R c b1 ->
@@ -907,6 +932,7 @@ Proof.
     move => [d h].
     exists d. hauto lq:on rew:off ctrs:EPar.R use:OExp.merge.
   - qauto use:Bot_EPar', EPar.refl.
+  - qauto use:Univ_EPar', EPar.refl.
 Qed.
 
 Function tstar {n} (a : Tm n) :=
@@ -923,6 +949,7 @@ Function tstar {n} (a : Tm n) :=
   | Proj p a => Proj p (tstar a)
   | Pi a b => Pi (tstar a) (tstar b)
   | Bot => Bot
+  | Univ i => Univ i
   end.
 
 Lemma RPar_triangle n (a : Tm n) : forall b, RPar.R a b -> RPar.R b (tstar a).
@@ -940,6 +967,7 @@ Proof.
   - hauto lq:on inv:RPar.R ctrs:RPar.R.
   - hauto lq:on inv:RPar.R ctrs:RPar.R.
   - hauto lq:on inv:RPar.R ctrs:RPar.R.
+  - hauto lq:on inv:RPar.R ctrs:RPar.R.
 Qed.
 
 Lemma RPar_diamond n (c a1 b1 : Tm n) :
@@ -973,6 +1001,7 @@ Fixpoint depth_tm {n} (a : Tm n) :=
   | Proj p a => 1 + depth_tm a
   | Pair a b => 1 + max (depth_tm a) (depth_tm b)
   | Bot => 1
+  | Univ i => 1
   end.
 
 Lemma depth_ren n m (ξ: fin n -> fin m) a :
@@ -1007,6 +1036,7 @@ Proof.
       by rewrite -depth_ren.
     + by simpl.
   - sfirstorder.
+  - sfirstorder.
 Qed.
 
 Lemma depth_subst_bool n (a : Tm (S n)) :
@@ -1019,6 +1049,7 @@ Qed.
 Local Ltac prov_tac := sfirstorder use:depth_ren.
 Local Ltac extract_tac := rewrite -?depth_subst_bool;hauto use:depth_subst_bool.
 
+(* Can consider combine prov and provU *)
 #[tactic="prov_tac"]Equations prov {n} (A : Tm n) (B : Tm (S n)) (a : Tm n) : Prop by wf (depth_tm a) lt :=
   prov A B (Pi A0 B0) := rtc Par.R A A0 /\ rtc Par.R B B0;
   prov A B (Abs a) := prov (ren_Tm shift A) (ren_Tm (upRen_Tm_Tm shift) B) a;
@@ -1026,7 +1057,18 @@ Local Ltac extract_tac := rewrite -?depth_subst_bool;hauto use:depth_subst_bool.
   prov A B (Pair a b) := prov A B a /\ prov A B b;
   prov A B (Proj p a) := prov A B a;
   prov A B Bot := False;
-  prov A B (VarTm _) := False.
+  prov A B (VarTm _) := False;
+  prov A B (Univ _) := False .
+
+#[tactic="prov_tac"]Equations provU {n} (i : nat) (a : Tm n) : Prop by wf (depth_tm a) lt :=
+  provU i (Pi A0 B0) := False;
+  provU i (Abs a) := provU i a;
+  provU i (App a b) := provU i a;
+  provU i (Pair a b) := provU i a /\ provU i b;
+  provU i (Proj p a) := provU i a;
+  provU i Bot := False;
+  provU i (VarTm _) := False;
+  provU i (Univ j) := i = j.
 
 #[tactic="prov_tac"]Equations extract {n} (a : Tm n) : Tm n by wf (depth_tm a) lt :=
   extract (Pi A B) := Pi A B;
@@ -1035,7 +1077,9 @@ Local Ltac extract_tac := rewrite -?depth_subst_bool;hauto use:depth_subst_bool.
   extract (Pair a b) := extract a;
   extract (Proj p a) := extract a;
   extract Bot := Bot;
-  extract (VarTm _) := Bot.
+  extract (VarTm i) := (VarTm i);
+  extract (Univ i) := Univ i.
+
 
 Lemma ren_extract n m (a : Tm n) (ξ : fin n -> fin m) :
   extract (ren_Tm ξ a) = ren_Tm ξ (extract a).
@@ -1051,6 +1095,7 @@ Proof.
   - hauto q:on rew:db:extract.
   - hauto q:on rew:db:extract.
   - sfirstorder.
+  - sfirstorder.
 Qed.
 
 Lemma tm_depth_ind (P : forall n, Tm n -> Prop)   :
@@ -1084,6 +1129,26 @@ Proof.
     simp prov.
     hauto l:on use:Pars.renaming.
   - sfirstorder.
+  - sfirstorder.
+Qed.
+
+Lemma provU_ren n m (ξ : fin n -> fin m) i a :
+  provU i a -> provU i (ren_Tm ξ a).
+Proof.
+  move : m ξ i. elim : n / a.
+  - sfirstorder rew:db:prov.
+  - move => n a ih m ξ i.
+    simp provU =>/=.
+    move /ih => {ih}.
+    move /(_ _ (upRen_Tm_Tm ξ)).
+    simp provU.
+  - hauto q:on rew:db:provU.
+  - qauto l:on rew:db:provU.
+  - hauto lq:on rew:db:provU.
+  - move => n A0 ih B0 h0 m ξ i /=.
+    simp prov.
+  - sfirstorder.
+  - sfirstorder.
 Qed.
 
 Lemma prov_morph n m (ρ : fin n -> Tm m) A B a :
@@ -1102,8 +1167,27 @@ Proof.
   - hauto lq:on rew:db:prov.
   - hauto l:on use:Pars.substing rew:db:prov.
   - qauto rew:db:prov.
+  - qauto rew:db:prov.
 Qed.
 
+Lemma provU_morph n m (ρ : fin n -> Tm m) i a :
+  provU i a ->
+  provU i (subst_Tm ρ a).
+Proof.
+  move : m ρ i. elim : n / a.
+  - hauto q:on rew:db:provU.
+  - move => n a ih m ρ i.
+    simp provU => /=.
+    move /ih => {ih}.
+    move /(_ _ (up_Tm_Tm ρ)). asimpl.
+    simp provU.
+  - hauto q:on rew:db:provU.
+  - hauto q:on rew:db:provU.
+  - hauto lq:on rew:db:provU.
+  - hauto l:on use:Pars.substing rew:db:provU.
+  - qauto rew:db:provU.
+  - qauto rew:db:provU.
+Qed.
 
 Lemma prov_par n (A : Tm n) B a b : prov A B a -> Par.R a b -> prov A B b.
 Proof.
@@ -1129,6 +1213,32 @@ Proof.
     move => [hA0 hA1].
     eauto using rtc_r.
   - sfirstorder.
+  - sfirstorder.
+Qed.
+
+Lemma provU_par n i (a b : Tm n) : provU i a -> Par.R a b -> provU i b.
+Proof.
+  move => + h. move : i.
+  elim : n a b /h.
+  - move => n a0 a1 b0 b1 ha iha hb ihb i /=.
+    simp provU => h.
+    move /iha : h.
+    move /(provU_morph _ _ (scons b1 VarTm)).
+    by asimpl.
+  - hauto lq:on rew:db:provU.
+  - hauto lq:on rew:db:provU.
+  - hauto lq:on rew:db:provU.
+  - move => n a0 a1 ha iha i. simp provU. move /iha.
+    hauto l:on use:provU_ren.
+  - hauto l:on rew:db:provU.
+  - simp provU.
+  - move => n a0 a1 ha iha i. simp provU.
+  - hauto l:on rew:db:provU.
+  - hauto l:on rew:db:provU.
+  - hauto lq:on rew:db:provU.
+  - move => n A0 A1 B0 B1 hA ihA hB ihB i. simp provU.
+  - sfirstorder.
+  - sfirstorder.
 Qed.
 
 Lemma prov_pars n (A : Tm n) B a b : prov A B a -> rtc Par.R a b -> prov A B b.
@@ -1136,6 +1246,11 @@ Proof.
   induction 2; hauto lq:on use:prov_par.
 Qed.
 
+Lemma provU_pars n i (a : Tm n) b : provU i a -> rtc Par.R a b -> provU i b.
+Proof.
+  induction 2; hauto lq:on use:provU_par.
+Qed.
+
 Lemma prov_extract n A B (a : Tm n) :
   prov A B a -> exists A0 B0,
       extract a = Pi A0 B0 /\ rtc Par.R A A0 /\ rtc Par.R B B0.
@@ -1164,6 +1279,26 @@ Proof.
   - qauto l:on rew:db:prov, extract.
 Qed.
 
+Lemma provU_extract n i (a : Tm n) :
+  provU i a -> extract a = Univ i.
+Proof.
+  move : i. elim : n / a => //=.
+  - move => n a ih i.
+    simp provU.
+    move /ih.
+    simp extract.
+    move => h.
+    (* anti renaming for par *)
+    have {}h0 : subst_Tm (scons Bot VarTm) (extract a) =
+    subst_Tm (scons Bot VarTm) (ren_Tm shift (Univ i)) by sauto lq:on.
+    move : h0. asimpl. move => ->.
+    hauto lq:on.
+  - hauto l:on rew:db:provU, extract.
+  - hauto l:on rew:db:provU, extract.
+  - hauto l:on rew:db:provU, extract.
+  - qauto l:on rew:db:provU, extract.
+Qed.
+
 Lemma EPar_Par n (a b : Tm n) : EPar.R a b -> Par.R a b.
 Proof.
   move => h. elim : n a b /h; qauto ctrs:Par.R.
@@ -1379,6 +1514,7 @@ Proof.
   - sfirstorder use:ERPars.ProjCong.
   - sfirstorder use:ERPars.PiCong.
   - sfirstorder.
+  - sfirstorder.
 Qed.
 
 Lemma Pars_ERPar n (a b : Tm n) : rtc Par.R a b -> rtc ERPar.R a b.
@@ -1550,6 +1686,15 @@ Proof. sfirstorder unfold:join. Qed.
 Lemma join_refl n (a : Tm n) : join a a.
 Proof. hauto lq:on ctrs:rtc unfold:join. Qed.
 
+Lemma pars_univ_inv n i (c : Tm n) :
+  rtc Par.R (Univ i) c ->
+  extract c = Univ i.
+Proof.
+  have : provU i (Univ i : Tm n) by sfirstorder.
+  move : provU_pars. repeat move/[apply].
+  by move/provU_extract.
+Qed.
+
 Lemma pars_pi_inv n (A : Tm n) B C :
   rtc Par.R (Pi A B) C ->
   exists A0 B0, extract C = Pi A0 B0 /\
@@ -1560,6 +1705,14 @@ Proof.
   by move /prov_extract.
 Qed.
 
+Lemma pars_univ_inj n i j (C : Tm n) :
+  rtc Par.R (Univ i) C ->
+  rtc Par.R (Univ j) C ->
+  i = j.
+Proof.
+  sauto l:on use:pars_univ_inv.
+Qed.
+
 Lemma pars_pi_inj n (A0 A1 : Tm n) B0 B1 C :
   rtc Par.R (Pi A0 B0) C ->
   rtc Par.R (Pi A1 B1) C ->
@@ -1571,6 +1724,12 @@ Proof.
   exists A2, B2. hauto l:on.
 Qed.
 
+Lemma join_univ_inj n i j (C : Tm n) :
+  join (Univ i : Tm n) (Univ j) -> i = j.
+Proof.
+  sfirstorder use:pars_univ_inj.
+Qed.
+
 Lemma join_pi_inj n (A0 A1 : Tm n) B0 B1 :
   join (Pi A0 B0) (Pi A1 B1) ->
   join A0 A1 /\ join B0 B1.
@@ -1579,3 +1738,13 @@ Proof.
   move : pars_pi_inj; repeat move/[apply].
   sfirstorder unfold:join.
 Qed.
+
+Lemma join_univ_pi_contra n (A : Tm n) B i :
+  join (Pi A B) (Univ i) -> False.
+Proof.
+  rewrite /join.
+  move => [c [h0 h1]].
+  move /pars_univ_inv : h1.
+  move /pars_pi_inv : h0.
+  hauto l:on.
+Qed.
diff --git a/theories/logrel.v b/theories/logrel.v
new file mode 100644
index 0000000..2c3a99e
--- /dev/null
+++ b/theories/logrel.v
@@ -0,0 +1,4 @@
+Require Import Autosubst2.core Autosubst2.fintype Autosubst2.syntax.
+Definition ProdSpace {n} (PA : Tm n -> Prop)
+  (PF : Tm n -> (Tm n -> Prop) -> Prop) : Tm n -> Prop :=
+  fun b => forall a PB, PA a -> PF a PB -> PB (App b a).