------------------------------------------------------------------------
-- A variant of the logical relation with a hidden reducibility
-- argument
------------------------------------------------------------------------

open import Definition.Typed.Restrictions
open import Graded.Erasure.LogicalRelation.Assumptions
open import Graded.Modality

module Graded.Erasure.LogicalRelation.Hidden
  {a} {M : Set a}
  {𝕄 : Modality M}
  {TR : Type-restrictions 𝕄}
  (as : Assumptions TR)
  where

open Assumptions as
open Modality 𝕄 hiding (_≤_; _<_)
open Type-restrictions TR

open import Definition.LogicalRelation.Simplified TR
open import Definition.Typed TR
open import Definition.Typed.Consequences.Inversion TR
open import Definition.Typed.Inversion TR
open import Definition.Typed.Properties TR
open import Definition.Typed.Substitution TR
open import Definition.Typed.Syntactic TR
open import Definition.Typed.Well-formed TR
open import Definition.Untyped M
open import Definition.Untyped.Properties M
open import Definition.Untyped.Whnf M type-variant

open import Graded.Context 𝕄
open import Graded.Context.Properties 𝕄
open import Graded.Erasure.Extraction 𝕄
open import Graded.Erasure.LogicalRelation as
open import Graded.Erasure.LogicalRelation.Conversion as
open import Graded.Erasure.LogicalRelation.Irrelevance as
open import Graded.Erasure.LogicalRelation.Reduction as
open import Graded.Erasure.Target as T using (strict)
import Graded.Erasure.Target.Properties as TP
open import Graded.Modality.Properties 𝕄
open import Graded.Mode 𝕄

open import Tools.Bool
open import Tools.Empty
open import Tools.Fin
open import Tools.Function
open import Tools.Level
open import Tools.Nat using (Nat; _<_)
open import Tools.Product as Σ
open import Tools.PropositionalEquality as PE using (_≢_)
import Tools.Reasoning.PropositionalEquality
open import Tools.Relation
open import Tools.Unit

private variable
  n              : Nat
  Γ              : Con Term _
  A B t t′ t₁ t₂ : Term _
  v v′           : T.Term _
  σ              : Subst _ _
  σ′             : T.Subst _ _
  γ δ            : Conₘ _
  p q            : M
  m              : Mode
  s              : Strength
  l l′           : Universe-level
  ok             : T _

------------------------------------------------------------------------
-- The type former

opaque

  -- A variant of _®_∷_/_.

  infix 19 _®_∷_

  _®_∷_ : Term k → T.Term k → Term k → Set a
  t ® v ∷ A =
    ∃ λ (⊨A : ts » Δ ⊨ A) → t ® v ∷ A / ⊨A

------------------------------------------------------------------------
-- Some characterisation lemmas for _®_∷_

opaque
  unfolding _®_∷_

  -- A characterisation lemma for U.

  ®∷U⇔ : t ® v ∷ U l ⇔ t ® v ∷U
  ®∷U⇔ {t} {v} {l} =
    t ® v ∷ U l                                   ⇔⟨ id⇔ ⟩
    (∃ λ (⊩U : ts » Δ ⊨ U l) → t ® v ∷ U l / ⊩U)  ⇔⟨ (λ (⊨U , t®v) → (irrelevanceTerm ⊨U) (U-intro ⊢Δ) t®v)
                                                   , (λ t®v → U-intro ⊢Δ , t®v)
                                                   ⟩
    (t ® v ∷U)                                    □⇔

opaque
  unfolding _®_∷_

  -- A characterisation lemma for Empty.

  ®∷Empty⇔ : t ® v ∷ Empty ⇔ t ® v ∷Empty
  ®∷Empty⇔ =
    (λ (⊩Empty′ , t®v) →
       irrelevanceTerm ⊩Empty′ (Empty-intro ⊢Δ) t®v)
    , (λ ())

opaque
  unfolding _®_∷_

  -- A characterisation lemma for Unit.

  ®∷Unit⇔ : t ® v ∷ Unit s l ⇔ t ® v ∷Unit⟨ s , l ⟩
  ®∷Unit⇔ =
      (λ (⊨Unit , t®v) →
        irrelevanceTerm ⊨Unit
          (Unit-intro ⊢Δ (inversion-Unit (⊨→⊢ ⊨Unit)))
          t®v)
         -- irrelevanceTerm ⊨Unit
           -- (Unitᵣ {!Unit-elim ⊨Unit!}) t®v)
    , (λ t®v → Unit-intro ⊢Δ (case t®v of λ {
                   (starᵣ t⇒* _) →
                   inversion-Unit (wf-⊢∷ (wf-⇛ t⇒* .proj₁)) })
               , t®v)

opaque
  unfolding _®_∷_

  -- A characterisation lemma for ℕ.

  ®∷ℕ⇔ : t ® v ∷ ℕ ⇔ t ® v ∷ℕ
  ®∷ℕ⇔ =
      (λ (⊨ℕ′ , t®v) →
         irrelevanceTerm ⊨ℕ′
           (ℕ-intro ⊢Δ) t®v)
    , (λ t®v → ℕ-intro ⊢Δ , t®v)

opaque
  unfolding _®_∷_

  -- A characterisation lemma for Id.

  ®∷Id⇔ :
    t ® v ∷ Id A t₁ t₂ ⇔
    (ts » Δ ⊢ A × t ® v ∷Id⟨ A ⟩⟨ t₁ ⟩⟨ t₂ ⟩)
  ®∷Id⇔ =
      (λ (⊨Id , t®v) →
        let ⊢A , ⊢t , ⊢u = inversion-Id (⊨→⊢ ⊨Id)
        in  ⊢A , ((irrelevanceTerm ⊨Id) (Id-intro ⊢A ⊢t ⊢u) t®v))
    , (λ (⊢A , t®v) →
         (case t®v of λ {
           (rflᵣ t⇒* _) →
         let ⊢A , ⊢t , ⊢u = inversion-Id (wf-⊢∷ (wf-⇛ t⇒* .proj₁))
         in  Id-intro ⊢A ⊢t ⊢u , t®v}))

opaque
  unfolding _®_∷_

  -- A characterisation lemma for Π.

  ®∷Π⇔ :
    t ® v ∷ Π p , q ▷ A ▹ B ⇔
    (ts » Δ ⊢ Π p , q ▷ A ▹ B ×
     (str PE.≡ strict → ∃ λ v′ → vs T.⊢ v ⇒* T.lam v′) ×
     (∀ t′ → ts » Δ ⊢ t′ ∷ A →
      (p PE.≡ 𝟘 → t ∘⟨ 𝟘 ⟩ t′ ® app-𝟘 str v ∷ B [ t′ ]₀) ×
      (p ≢ 𝟘 →
       ∀ v′ → t′ ® v′ ∷ A →
       t ∘⟨ p ⟩ t′ ® v T.∘⟨ str ⟩ v′ ∷ B [ t′ ]₀)))
  ®∷Π⇔ {p} {B} =
      (λ (⊨Π , t®v′) →
        let ⊨A , ⊨B = ΠΣ-elim ⊨Π
            ⊢Π = ⊨→⊢ ⊨Π
            ⊢A , ⊢B , ok = inversion-ΠΣ ⊢Π
            t®v = irrelevanceTerm ⊨Π (ΠΣ-intro′ ⊨A ⊨B ⊢B ok) t®v′
        in  ⊢Π , t®v .proj₁
          , λ t′ ⊢t′ →
              (λ {PE.refl → ⊨B ⊢t′ , Π-®-𝟘 (is-𝟘? 𝟘) (t®v .proj₂ ⊢t′)})
            , λ p≢𝟘 _ t′®v′ →
                ⊨B ⊢t′
              , Π-®-ω p≢𝟘 (is-𝟘? p) (t®v .proj₂ ⊢t′)
                    (irrelevanceTerm (t′®v′ .proj₁)
                      ⊨A (t′®v′ .proj₂)))
      , (λ (⊢Π , v⇒*lam , t®v) →
           ΠΣ-intro ⊢Π , v⇒*lam , λ ⊢t′ → lemma (is-𝟘? p) t®v ⊢t′)
      where
      lemma :
        {⊨A : ts » Δ ⊨ _} {⊨B : ts » Δ ⊨ _}
        (d : Dec (p PE.≡ 𝟘)) →
        (∀ t′ → ts » Δ ⊢ t′ ∷ A →
         (p PE.≡ 𝟘 → t ∘⟨ 𝟘 ⟩ t′ ® app-𝟘 str v ∷ B [ t′ ]₀) ×
         (p ≢ 𝟘 →
          ∀ v′ → t′ ® v′ ∷ A →
          t ∘⟨ p ⟩ t′ ® v T.∘⟨ str ⟩ v′ ∷ B [ t′ ]₀)) →
        ts » Δ ⊢ t′ ∷ A →
        Π-® A B t t′ v ⊨A ⊨B p d
      lemma {⊨B} (yes PE.refl) t®v ⊢t′ =
        let ⊨B′ , tt′®v = t®v _ ⊢t′ .proj₁ PE.refl
        in  irrelevanceTerm ⊨B′ ⊨B tt′®v
      lemma {⊨B} (no p≢𝟘) t®v ⊢t′ t′®v′ =
        let ⊨B′ , tt′®vv′ = t®v _ ⊢t′ .proj₂ p≢𝟘 _ (_ , t′®v′ )
        in  irrelevanceTerm ⊨B′ ⊨B tt′®vv′

opaque

  -- A characterisation lemma for non-erased Π.

  ®∷Πω⇔ :
    p ≢ 𝟘 →
    t ® v ∷ Π p , q ▷ A ▹ B ⇔
    (ts » Δ ⊢ Π p , q ▷ A ▹ B ×
     (str PE.≡ strict → ∃ λ v′ → vs T.⊢ v ⇒* T.lam v′) ×
     (∀ t′ v′ → ts » Δ ⊢ t′ ∷ A → t′ ® v′ ∷ A →
      t ∘⟨ p ⟩ t′ ® v T.∘⟨ str ⟩ v′ ∷ B [ t′ ]₀))
  ®∷Πω⇔ {p} {t} {v} {q} {A} {B} p≢𝟘 =
    t ® v ∷ Π p , q ▷ A ▹ B                                ⇔⟨ ®∷Π⇔ ⟩

    ts » Δ ⊢ Π p , q ▷ A ▹ B ×
    (str PE.≡ strict → ∃ λ v′ → vs T.⊢ v ⇒* T.lam v′) ×
    (∀ t′ → ts » Δ ⊢ t′ ∷ A →
     (p PE.≡ 𝟘 → t ∘⟨ 𝟘 ⟩ t′ ® app-𝟘 str v ∷ B [ t′ ]₀) ×
     (p ≢ 𝟘 →
      ∀ v′ → t′ ® v′ ∷ A →
      t ∘⟨ p ⟩ t′ ® v T.∘⟨ str ⟩ v′ ∷ B [ t′ ]₀))          ⇔⟨ (Σ-cong-⇔ λ _ → Σ-cong-⇔ λ _ → Π-cong-⇔ λ _ →
                                                                 (λ hyp v′ ⊢t′ → hyp ⊢t′ .proj₂ p≢𝟘 v′)
                                                               , (λ hyp ⊢t′ → ⊥-elim ∘→ p≢𝟘 , λ _ v′ → hyp v′ ⊢t′)) ⟩
    ts » Δ ⊢ Π p , q ▷ A ▹ B ×
    (str PE.≡ strict → ∃ λ v′ → vs T.⊢ v ⇒* T.lam v′) ×
    (∀ t′ v′ → ts » Δ ⊢ t′ ∷ A → t′ ® v′ ∷ A →
     t ∘⟨ p ⟩ t′ ® v T.∘⟨ str ⟩ v′ ∷ B [ t′ ]₀)            □⇔

opaque

  -- A characterisation lemma for erased Π.

  ®∷Π₀⇔ :
    t ® v ∷ Π 𝟘 , q ▷ A ▹ B ⇔
    (ts » Δ ⊢ Π 𝟘 , q ▷ A ▹ B ×
     (str PE.≡ strict → ∃ λ v′ → vs T.⊢ v ⇒* T.lam v′) ×
     (∀ t′ → ts » Δ ⊢ t′ ∷ A → t ∘⟨ 𝟘 ⟩ t′ ® app-𝟘 str v ∷ B [ t′ ]₀))
  ®∷Π₀⇔ {t} {v} {q} {A} {B} =
    t ® v ∷ Π 𝟘 , q ▷ A ▹ B                                           ⇔⟨ ®∷Π⇔ ⟩

    ts » Δ ⊢ Π 𝟘 , q ▷ A ▹ B ×
    (str PE.≡ strict → ∃ λ v′ → vs T.⊢ v ⇒* T.lam v′) ×
    (∀ t′ → ts » Δ ⊢ t′ ∷ A →
     (𝟘 PE.≡ 𝟘 → t ∘⟨ 𝟘 ⟩ t′ ® app-𝟘 str v ∷ B [ t′ ]₀) ×
     (𝟘 ≢ 𝟘 →
      ∀ v′ → t′ ® v′ ∷ A →
      t ∘⟨ 𝟘 ⟩ t′ ® v T.∘⟨ str ⟩ v′ ∷ B [ t′ ]₀))                     ⇔⟨ (Σ-cong-⇔ λ _ → Σ-cong-⇔ λ _ → Π-cong-⇔ λ _ → Π-cong-⇔ λ _ →
                                                                            (λ hyp → hyp .proj₁ PE.refl)
                                                                          , (λ hyp → (λ _ → hyp) , ⊥-elim ∘→ (_$ PE.refl))) ⟩
    ts » Δ ⊢ Π 𝟘 , q ▷ A ▹ B ×
    (str PE.≡ strict → ∃ λ v′ → vs T.⊢ v ⇒* T.lam v′) ×
    (∀ t′ → ts » Δ ⊢ t′ ∷ A → t ∘⟨ 𝟘 ⟩ t′ ® app-𝟘 str v ∷ B [ t′ ]₀)  □⇔

opaque
  unfolding _®_∷_

  -- A characterisation lemma for Σ.

  ®∷Σ⇔ :
    t ® v ∷ Σ⟨ s ⟩ p , q ▷ A ▹ B ⇔
    (ts » Δ ⊢ Σ⟨ s ⟩ p , q ▷ A ▹ B ×
     ∃₃ λ t₁ t₂ v₂ →
     t ⇛ prod s p t₁ t₂ ∷ Σ⟨ s ⟩ p , q ▷ A ▹ B ×
     t₂ ® v₂ ∷ B [ t₁ ]₀ ×
     (p PE.≡ 𝟘 → vs T.⊢ v ⇒* v₂) ×
     (p ≢ 𝟘 → ∃ λ v₁ → vs T.⊢ v ⇒* T.prod v₁ v₂ × t₁ ® v₁ ∷ A))
  ®∷Σ⇔ {t} {v} {s} {p} {q} {A} {B} =
      (λ (⊨Σ , t®v′) →
        let ⊨A , ⊨B = ΠΣ-elim ⊨Σ
            ⊢Σ = ⊨→⊢ ⊨Σ
            ⊢A , ⊢B , ok = inversion-ΠΣ ⊢Σ
            t₁ , t₂ , t⇒ , ⊢t₁ , v₂ , t₂®v₂ , rest = irrelevanceTerm ⊨Σ (ΠΣ-intro′ ⊨A ⊨B ⊢B ok) t®v′
        in  ⊢Σ , t₁ , t₂ , v₂ , t⇒ , (_ , t₂®v₂)
               , (λ { PE.refl → Σ-®-𝟘 rest })
               , λ p≢𝟘 →
                 let v₁ , v⇒ , t₁®v₁ = Σ-®-ω p≢𝟘 rest
                 in  v₁ , v⇒ , _ , t₁®v₁)
      , λ (⊢Σ , _ , _ , v₂ , t⇒*prod , (⊨B′ , t₂®v₂) , hyp₁ , hyp₂) →
        let ⊢A , ⊢B , ok = inversion-ΠΣ ⊢Σ
            ⊨A = ⊢→⊨ ⊢A
            ⊨Σ = ΠΣ-intro′ ⊨A (λ ⊢t → ⊢→⊨ (substType ⊢B ⊢t)) ⊢B ok
            ⊢t₁ = inversion-prod-Σ (wf-⇛ t⇒*prod .proj₂) .proj₁
        in  ⊨Σ
          , _ , _ , t⇒*prod , ⊢t₁ , _
          , irrelevanceTerm _ (⊢→⊨ (substType ⊢B ⊢t₁)) t₂®v₂
          , (case is-𝟘? p of λ where
              (yes PE.refl) → Σ-®-intro-𝟘 (hyp₁ PE.refl) PE.refl
              (no p≢𝟘) →
                case hyp₂ p≢𝟘 of λ
                    (v₁ , v⇒*prod , (⊨A′ , t₁®v₁)) →
                  Σ-®-intro-ω v₁ v⇒*prod
                    (irrelevanceTerm ⊨A′ ⊨A t₁®v₁)
                    p≢𝟘)

opaque

  -- A characterisation lemma for non-erased Σ.

  ®∷Σω⇔ :
    p ≢ 𝟘 →
    t ® v ∷ Σ⟨ s ⟩ p , q ▷ A ▹ B ⇔
    (ts » Δ ⊢ Σ⟨ s ⟩ p , q ▷ A ▹ B ×
     ∃₄ λ t₁ t₂ v₁ v₂ →
     t ⇛ prod s p t₁ t₂ ∷ Σ⟨ s ⟩ p , q ▷ A ▹ B ×
     vs T.⊢ v ⇒* T.prod v₁ v₂ ×
     t₁ ® v₁ ∷ A ×
     t₂ ® v₂ ∷ B [ t₁ ]₀)
  ®∷Σω⇔ {p} {t} {v} {s} {q} {A} {B} p≢𝟘 =
    t ® v ∷ Σ⟨ s ⟩ p , q ▷ A ▹ B                                 ⇔⟨ ®∷Σ⇔ ⟩

    (ts » Δ ⊢ Σ⟨ s ⟩ p , q ▷ A ▹ B ×
     ∃₃ λ t₁ t₂ v₂ →
     t ⇛ prod s p t₁ t₂ ∷ Σ⟨ s ⟩ p , q ▷ A ▹ B ×
     t₂ ® v₂ ∷ B [ t₁ ]₀ ×
     (p PE.≡ 𝟘 → vs T.⊢ v ⇒* v₂) ×
     (p ≢ 𝟘 → ∃ λ v₁ → vs T.⊢ v ⇒* T.prod v₁ v₂ × t₁ ® v₁ ∷ A))  ⇔⟨ (Σ-cong-⇔ λ _ → Σ-cong-⇔ λ _ → Σ-cong-⇔ λ _ →
                                                                       (λ (v₂ , t⇒* , t₂®v₂ , _ , hyp) →
                                                                          case hyp p≢𝟘 of λ
                                                                            (v₁ , v⇒* , t₁®v₁) →
                                                                          v₁ , v₂ , t⇒* , v⇒* , t₁®v₁ , t₂®v₂)
                                                                     , (λ (v₁ , v₂ , t⇒* , v⇒* , t₁®v₁ , t₂®v₂) →
                                                                          v₂ , t⇒* , t₂®v₂ , ⊥-elim ∘→ p≢𝟘 , λ _ → v₁ , v⇒* , t₁®v₁)) ⟩
    (ts » Δ ⊢ Σ⟨ s ⟩ p , q ▷ A ▹ B ×
     ∃₄ λ t₁ t₂ v₁ v₂ →
     t ⇛ prod s p t₁ t₂ ∷ Σ⟨ s ⟩ p , q ▷ A ▹ B ×
     vs T.⊢ v ⇒* T.prod v₁ v₂ ×
     t₁ ® v₁ ∷ A ×
     t₂ ® v₂ ∷ B [ t₁ ]₀)                                        □⇔

opaque

  -- A characterisation lemma for erased Σ.

  ®∷Σ₀⇔ :
    t ® v ∷ Σ⟨ s ⟩ 𝟘 , q ▷ A ▹ B ⇔
    (ts » Δ ⊢ Σ⟨ s ⟩ 𝟘 , q ▷ A ▹ B ×
     ∃₃ λ t₁ t₂ v′ →
     t ⇛ prod s 𝟘 t₁ t₂ ∷ Σ⟨ s ⟩ 𝟘 , q ▷ A ▹ B ×
     vs T.⊢ v ⇒* v′ ×
     t₂ ® v′ ∷ B [ t₁ ]₀)
  ®∷Σ₀⇔ {t} {v} {s} {q} {A} {B} =
    t ® v ∷ Σ⟨ s ⟩ 𝟘 , q ▷ A ▹ B                                 ⇔⟨ ®∷Σ⇔ ⟩

    (ts » Δ ⊢ Σ⟨ s ⟩ 𝟘 , q ▷ A ▹ B ×
     ∃₃ λ t₁ t₂ v₂ →
     t ⇛ prod s 𝟘 t₁ t₂ ∷ Σ⟨ s ⟩ 𝟘 , q ▷ A ▹ B ×
     t₂ ® v₂ ∷ B [ t₁ ]₀ ×
     (𝟘 PE.≡ 𝟘 → vs T.⊢ v ⇒* v₂) ×
     (𝟘 ≢ 𝟘 → ∃ λ v₁ → vs T.⊢ v ⇒* T.prod v₁ v₂ × t₁ ® v₁ ∷ A))  ⇔⟨ (Σ-cong-⇔ λ _ → Σ-cong-⇔ λ _ → Σ-cong-⇔ λ _ → Σ-cong-⇔ λ _ → Σ-cong-⇔ λ _ →
                                                                       (λ (t₂®v₂ , hyp , _) → hyp PE.refl , t₂®v₂)
                                                                     , (λ (v⇒* , t₂®v₂) → t₂®v₂ , (λ _ → v⇒*) , ⊥-elim ∘→ (_$ PE.refl))) ⟩
    (ts » Δ ⊢ Σ⟨ s ⟩ 𝟘 , q ▷ A ▹ B ×
     ∃₃ λ t₁ t₂ v′ →
     t ⇛ prod s 𝟘 t₁ t₂ ∷ Σ⟨ s ⟩ 𝟘 , q ▷ A ▹ B ×
     vs T.⊢ v ⇒* v′ ×
     t₂ ® v′ ∷ B [ t₁ ]₀)                                        □⇔

------------------------------------------------------------------------
-- The type formers _®_∷_◂_, Definitions-related, _®_∷[_∣_]_◂_,
-- _®_∷[_]_◂_, _▸_⊩ʳ_∷[_∣_]_ and _▸_⊩ʳ_∷[_]_

opaque

  -- A variant of _®_∷_ that is trivial (up to _⇔_) when the last
  -- argument is 𝟘.

  infix 19 _®_∷_◂_

  _®_∷_◂_ : Term k → T.Term k → Term k → M → Set a
  t ® v ∷ A ◂ p = p ≢ 𝟘 → t ® v ∷ A

opaque

  -- Definitions-related m n means that the first n definitions in ts
  -- and vs are related to each other in a certain way.

  Definitions-related : Mode → Nat → Set a
  Definitions-related m n =
    ∀ {α t A v} → α < n → α ↦ t ∷ A ∈ ts → α T.↦ v ∈ vs →
    wk wk₀ t ® T.wk wk₀ v ∷ wk wk₀ A ◂ ⌜ m ⌝

opaque

  -- A logical relation for substitutions. If the mode is m and the
  -- natural number (the one that is an explicit argument) is n, then
  -- it is required that Definitions-related m n holds.

  infix 19 _®_∷[_∣_]_◂_

  _®_∷[_∣_]_◂_ :
    Subst k n → T.Subst k n → Mode → Nat → Con Term n → Conₘ n → Set a
  _ ® _  ∷[ m ∣ n ] ε     ◂ ε     = Definitions-related m n
  σ ® σ′ ∷[ m ∣ n ] Γ ∙ A ◂ γ ∙ p =
    head σ ® T.head σ′ ∷ A [ tail σ ] ◂ ⌜ m ⌝ · p ×
    tail σ ® T.tail σ′ ∷[ m ∣ n ] Γ ◂ γ

-- A logical relation for substitutions.

infix 19 _®_∷[_]_◂_

_®_∷[_]_◂_ :
  Subst k n → T.Subst k n → Mode → Con Term n → Conₘ n → Set a
σ ® σ′ ∷[ m ] Γ ◂ γ = σ ® σ′ ∷[ m ∣ 0 ] Γ ◂ γ

opaque

  -- Validity with respect to erasure (assuming that a certain number
  -- of definitions are "OK").

  infix 19 _▸_⊩ʳ_∷[_∣_]_

  _▸_⊩ʳ_∷[_∣_]_ :
    Conₘ n → Con Term n → Term n → Mode → Nat → Term n → Set a
  γ ▸ Γ ⊩ʳ t ∷[ m ∣ n ] A =
    ∀ {σ σ′} →
    ts » Δ ⊢ˢʷ σ ∷ Γ →
    σ ® σ′ ∷[ m ∣ n ] Γ ◂ γ →
    t [ σ ] ® erase str t T.[ σ′ ] ∷ A [ σ ] ◂ ⌜ m ⌝

-- Validity with respect to erasure.

infix 19 _▸_⊩ʳ_∷[_]_

_▸_⊩ʳ_∷[_]_ : Conₘ n → Con Term n → Term n → Mode → Term n → Set a
γ ▸ Γ ⊩ʳ t ∷[ m ] A = γ ▸ Γ ⊩ʳ t ∷[ m ∣ 0 ] A

------------------------------------------------------------------------
-- Characterisation lemmas for _®_∷_◂_, Definitions-related,
-- _®_∷[_∣_]_◂_ and _▸_⊩ʳ_∷[_∣_]_

opaque
  unfolding _®_∷_◂_

  -- A characterisation lemma for _®_∷_◂_.

  ®∷◂⇔ : t ® v ∷ A ◂ p ⇔ (p ≢ 𝟘 → t ® v ∷ A)
  ®∷◂⇔ = id⇔

opaque
  unfolding Definitions-related

  -- A characterisation lemma for Definitions-related.

  Definitions-related⇔ :
    Definitions-related m n ⇔
    (∀ {α t A v} → α < n → α ↦ t ∷ A ∈ ts → α T.↦ v ∈ vs →
     wk wk₀ t ® T.wk wk₀ v ∷ wk wk₀ A ◂ ⌜ m ⌝)
  Definitions-related⇔ = id⇔

opaque
  unfolding _®_∷[_∣_]_◂_

  -- A characterisation lemma for _®_∷[_∣_]_◂_.

  ®∷[∣]ε◂ε⇔ : σ ® σ′ ∷[ m ∣ n ] ε ◂ ε ⇔ Definitions-related m n
  ®∷[∣]ε◂ε⇔ = id⇔

opaque
  unfolding _®_∷[_∣_]_◂_

  -- Another characterisation lemma for _®_∷[_∣_]_◂_.

  ®∷[∣]∙◂∙⇔ :
    σ ® σ′ ∷[ m ∣ n ] Γ ∙ A ◂ γ ∙ p ⇔
    (head σ ® T.head σ′ ∷ A [ tail σ ] ◂ ⌜ m ⌝ · p ×
     tail σ ® T.tail σ′ ∷[ m ∣ n ] Γ ◂ γ)
  ®∷[∣]∙◂∙⇔ = id⇔

opaque

  -- Yet another characterisation lemma for _®_∷[_∣_]_◂_.

  ®∷[∣]◂⇔ :
    σ ® σ′ ∷[ m ∣ n ] Γ ◂ γ ⇔
    (Definitions-related m n ×
     (∀ {A x} → x ∷ A ∈ Γ →
      σ x ® σ′ x ∷ A [ σ ] ◂ ⌜ m ⌝ · γ ⟨ x ⟩))
  ®∷[∣]◂⇔ {σ} {σ′} {m} {n} {Γ = ε} {γ = ε} =
    σ ® σ′ ∷[ m ∣ n ] ε ◂ ε                                 ⇔⟨ ®∷[∣]ε◂ε⇔ ⟩

    Definitions-related m n                                 ⇔⟨ (_, (λ ())) , proj₁ ⟩

    Definitions-related m n ×
    (∀ {A x} →
     x ∷ A ∈ ε →
     σ x ® σ′ x ∷ A [ σ ] ◂ ⌜ m ⌝ · ε ⟨ x ⟩)                □⇔
  ®∷[∣]◂⇔ {σ} {σ′} {m} {n} {Γ = Γ ∙ A} {γ = γ ∙ p} =
    σ ® σ′ ∷[ m ∣ n ] Γ ∙ A ◂ γ ∙ p                                 ⇔⟨ ®∷[∣]∙◂∙⇔ ⟩

    head σ ® T.head σ′ ∷ A [ tail σ ] ◂ ⌜ m ⌝ · p ×
    tail σ ® T.tail σ′ ∷[ m ∣ n ] Γ ◂ γ                             ⇔⟨ id⇔ ×-cong-⇔ ®∷[∣]◂⇔ ⟩

    head σ ® T.head σ′ ∷ A [ tail σ ] ◂ ⌜ m ⌝ · p ×
    Definitions-related m n ×
    (∀ {B x} → x ∷ B ∈ Γ →
     tail σ x ® T.tail σ′ x ∷ B [ tail σ ] ◂ ⌜ m ⌝ · γ ⟨ x ⟩)       ⇔⟨ (λ (hyp₁ , hyp₂ , hyp₃) → hyp₂ , hyp₁ , hyp₃)
                                                                     , (λ (hyp₁ , hyp₂ , hyp₃) → hyp₂ , hyp₁ , hyp₃)
                                                                     ⟩
    Definitions-related m n ×
    head σ ® T.head σ′ ∷ A [ tail σ ] ◂ ⌜ m ⌝ · p ×
    (∀ {B x} → x ∷ B ∈ Γ →
     tail σ x ® T.tail σ′ x ∷ B [ tail σ ] ◂ ⌜ m ⌝ · γ ⟨ x ⟩)       ⇔⟨ id⇔
                                                                         ×-cong-⇔
                                                                       PE.subst (flip _⇔_ _)
                                                                         (PE.cong₂ (_®_∷_◂_ _ _) (wk1-tail A) PE.refl) id⇔
                                                                         ×-cong-⇔
                                                                       (implicit-Π-cong-⇔ λ B → implicit-Π-cong-⇔ λ _ →
                                                                        Π-cong-⇔ λ _ →
                                                                        PE.subst (flip _⇔_ _)
                                                                          (PE.cong₂ (_®_∷_◂_ _ _) (wk1-tail B) PE.refl) id⇔) ⟩
    Definitions-related m n ×
    head σ ® T.head σ′ ∷ wk1 A [ σ ] ◂ ⌜ m ⌝ · p ×
    (∀ {B x} → x ∷ B ∈ Γ →
     tail σ x ® T.tail σ′ x ∷ wk1 B [ σ ] ◂ ⌜ m ⌝ · γ ⟨ x ⟩)        ⇔⟨ id⇔
                                                                         ×-cong-⇔
                                                                       ( (λ (hyp₁ , hyp₂) {_ _} → λ { here → hyp₁; (there x∈) → hyp₂ x∈ })
                                                                       , (λ hyp → hyp here , λ {_ _} → hyp ∘→ there)
                                                                       )
                                                                     ⟩
    Definitions-related m n ×
    (∀ {B x} →
     x ∷ B ∈ Γ ∙ A →
     σ x ® σ′ x ∷ B [ σ ] ◂ ⌜ m ⌝ · (γ ∙ p) ⟨ x ⟩)                  □⇔

opaque
  unfolding _▸_⊩ʳ_∷[_∣_]_

  -- A characterisation lemma for _▸_⊩ʳ_∷[_∣_]_.

  ▸⊩ʳ∷⇔ :
    γ ▸ Γ ⊩ʳ t ∷[ m ∣ n ] A ⇔
    (∀ {σ σ′} → ts » Δ ⊢ˢʷ σ ∷ Γ → σ ® σ′ ∷[ m ∣ n ] Γ ◂ γ →
     t [ σ ] ® erase str t T.[ σ′ ] ∷ A [ σ ] ◂ ⌜ m ⌝)
  ▸⊩ʳ∷⇔ = id⇔

------------------------------------------------------------------------
-- Some subsumption lemmas

opaque

  -- Subsumption for _®_∷_◂_.

  subsumption-®∷◂ :
    (p PE.≡ 𝟘 → q PE.≡ 𝟘) →
    t ® v ∷ A ◂ p →
    t ® v ∷ A ◂ q
  subsumption-®∷◂ {p} {q} {t} {v} {A} hyp =
    t ® v ∷ A ◂ p        ⇔⟨ ®∷◂⇔ ⟩→
    (p ≢ 𝟘 → t ® v ∷ A)  →⟨ _∘→ (_∘→ hyp) ⟩
    (q ≢ 𝟘 → t ® v ∷ A)  ⇔˘⟨ ®∷◂⇔ ⟩→
    t ® v ∷ A ◂ q        □

opaque

  -- Subsumption for _®_∷[_∣_]_◂_.

  subsumption-®∷[∣]◂ :
    (∀ x → γ ⟨ x ⟩ PE.≡ 𝟘 → δ ⟨ x ⟩ PE.≡ 𝟘) →
    σ ® σ′ ∷[ m ∣ n ] Γ ◂ γ →
    σ ® σ′ ∷[ m ∣ n ] Γ ◂ δ
  subsumption-®∷[∣]◂ {γ = ε} {δ = ε} {σ} {σ′} {m} {n} {Γ = ε} _ =
    σ ® σ′ ∷[ m ∣ n ] ε ◂ ε  □
  subsumption-®∷[∣]◂
    {γ = γ ∙ p} {δ = δ ∙ q} {σ} {σ′} {m} {n} {Γ = Γ ∙ A} hyp =
    σ ® σ′ ∷[ m ∣ n ] Γ ∙ A ◂ γ ∙ p                  ⇔⟨ ®∷[∣]∙◂∙⇔ ⟩→

    head σ ® T.head σ′ ∷ A [ tail σ ] ◂ ⌜ m ⌝ · p ×
    tail σ ® T.tail σ′ ∷[ m ∣ n ] Γ ◂ γ              →⟨ Σ.map
                                                          (subsumption-®∷◂ lemma)
                                                          (subsumption-®∷[∣]◂ (hyp ∘→ _+1)) ⟩
    head σ ® T.head σ′ ∷ A [ tail σ ] ◂ ⌜ m ⌝ · q ×
    tail σ ® T.tail σ′ ∷[ m ∣ n ] Γ ◂ δ              ⇔˘⟨ ®∷[∣]∙◂∙⇔ ⟩→

    σ ® σ′ ∷[ m ∣ n ] Γ ∙ A ◂ δ ∙ q                  □
    where
    lemma : ⌜ m ⌝ · p PE.≡ 𝟘 → ⌜ m ⌝ · q PE.≡ 𝟘
    lemma = case PE.singleton m of λ where
      (𝟘ᵐ , PE.refl) →
        𝟘 · p PE.≡ 𝟘  →⟨ (λ _ → ·-zeroˡ _) ⟩
        𝟘 · q PE.≡ 𝟘  □
      (𝟙ᵐ , PE.refl) →
        𝟙 · p PE.≡ 𝟘  ≡⟨ PE.cong (PE._≡ _) $ ·-identityˡ _ ⟩→
        p PE.≡ 𝟘      →⟨ hyp x0 ⟩
        q PE.≡ 𝟘      ≡⟨ PE.cong (PE._≡ _) $ PE.sym $ ·-identityˡ _ ⟩→
        𝟙 · q PE.≡ 𝟘  □

opaque

  -- Subsumption for _▸_⊩ʳ_∷[_∣_]_.

  subsumption-▸⊩ʳ∷[] :
    (∀ x → δ ⟨ x ⟩ PE.≡ 𝟘 → γ ⟨ x ⟩ PE.≡ 𝟘) →
    γ ▸ Γ ⊩ʳ t ∷[ m ∣ n ] A →
    δ ▸ Γ ⊩ʳ t ∷[ m ∣ n ] A
  subsumption-▸⊩ʳ∷[] {δ} {γ} {Γ} {t} {m} {n} {A} hyp =
    γ ▸ Γ ⊩ʳ t ∷[ m ∣ n ] A                                   ⇔⟨ ▸⊩ʳ∷⇔ ⟩→

    (∀ {σ σ′} → ts » Δ ⊢ˢʷ σ ∷ Γ → σ ® σ′ ∷[ m ∣ n ] Γ ◂ γ →
     t [ σ ] ® erase str t T.[ σ′ ] ∷ A [ σ ] ◂ ⌜ m ⌝)        →⟨ (_∘→ subsumption-®∷[∣]◂ hyp) ∘→_ ⟩

    (∀ {σ σ′} → ts » Δ ⊢ˢʷ σ ∷ Γ → σ ® σ′ ∷[ m ∣ n ] Γ ◂ δ →
     t [ σ ] ® erase str t T.[ σ′ ] ∷ A [ σ ] ◂ ⌜ m ⌝)        ⇔˘⟨ ▸⊩ʳ∷⇔ ⟩→

    δ ▸ Γ ⊩ʳ t ∷[ m ∣ n ] A                                   □

opaque

  -- Another kind of subsumption for _▸_⊩ʳ_∷[_∣_]_.

  subsumption-▸⊩ʳ∷[]-≤ :
    ⦃ 𝟘-well-behaved : Has-well-behaved-zero M semiring-with-meet ⦄ →
    δ ≤ᶜ γ →
    γ ▸ Γ ⊩ʳ t ∷[ m ∣ n ] A →
    δ ▸ Γ ⊩ʳ t ∷[ m ∣ n ] A
  subsumption-▸⊩ʳ∷[]-≤ δ≤γ =
    subsumption-▸⊩ʳ∷[] (λ _ → ≤ᶜ→⟨⟩≡𝟘→⟨⟩≡𝟘 δ≤γ)

------------------------------------------------------------------------
-- Some lemmas related to grades or modes

opaque

  -- If t ® v ∷ A holds, then t ® v ∷ A ◂ p holds for any p.

  ®∷→®∷◂ :
    t ® v ∷ A →
    t ® v ∷ A ◂ p
  ®∷→®∷◂ t®v = ®∷◂⇔ .proj₂ λ _ → t®v

opaque

  -- If t ® v ∷ A ◂ p holds for some non-zero p, then t ® v ∷ A holds.

  ®∷→®∷◂ω :
    p ≢ 𝟘 →
    t ® v ∷ A ◂ p →
    t ® v ∷ A
  ®∷→®∷◂ω {p} {t} {v} {A} p≢𝟘 =
    t ® v ∷ A ◂ p        ⇔⟨ ®∷◂⇔ ⟩→
    (p ≢ 𝟘 → t ® v ∷ A)  →⟨ _$ p≢𝟘 ⟩
    t ® v ∷ A            □

opaque

  -- If p is zero, then t ® v ∷ A ◂ p holds.

  ®∷◂𝟘 : p PE.≡ 𝟘 → t ® v ∷ A ◂ p
  ®∷◂𝟘 p≡𝟘 = ®∷◂⇔ .proj₂ (⊥-elim ∘→ (_$ p≡𝟘))

opaque

  -- The type γ ▸ Γ ⊩ʳ t ∷[ 𝟘ᵐ[ ok ] ∣ n ] A is inhabited.

  ▸⊩ʳ∷[𝟘ᵐ] : γ ▸ Γ ⊩ʳ t ∷[ 𝟘ᵐ[ ok ] ∣ n ] A
  ▸⊩ʳ∷[𝟘ᵐ] = ▸⊩ʳ∷⇔ .proj₂ (λ _ _ → ®∷◂𝟘 PE.refl)

------------------------------------------------------------------------
-- Some lemmas related to substitutions

opaque

  -- If Definitions-related m n holds, then σ ® σ′ ∷[ m ] Γ ◂ γ
  -- implies σ ® σ′ ∷[ m ∣ n ] Γ ◂ γ.

  ®∷[]◂→®∷[∣]◂ :
    Definitions-related m n →
    σ ® σ′ ∷[ m ] Γ ◂ γ →
    σ ® σ′ ∷[ m ∣ n ] Γ ◂ γ
  ®∷[]◂→®∷[∣]◂ defs-ok σ®σ′ =
    ®∷[∣]◂⇔ .proj₂ (defs-ok , ®∷[∣]◂⇔ .proj₁ σ®σ′ .proj₂)

opaque

  -- If Definitions-related m n holds, then γ ▸ Γ ⊩ʳ t ∷[ m ∣ n ] A
  -- implies γ ▸ Γ ⊩ʳ t ∷[ m ] A.

  ▸⊩ʳ∷[∣]→▸⊩ʳ∷ :
    Definitions-related m n →
    γ ▸ Γ ⊩ʳ t ∷[ m ∣ n ] A →
    γ ▸ Γ ⊩ʳ t ∷[ m ] A
  ▸⊩ʳ∷[∣]→▸⊩ʳ∷ defs-ok ⊩r =
    ▸⊩ʳ∷⇔ .proj₂ (λ ⊢σ → ▸⊩ʳ∷⇔ .proj₁ ⊩r ⊢σ ∘→ ®∷[]◂→®∷[∣]◂ defs-ok)

opaque

  -- If Definitions-related m n holds, then a source substitution is
  -- related to every (matching) target substitution with respect to
  -- the (matching) zero usage context.

  ®∷[∣]◂𝟘ᶜ : Definitions-related m n → σ ® σ′ ∷[ m ∣ n ] Γ ◂ 𝟘ᶜ
  ®∷[∣]◂𝟘ᶜ {m} defs-ok =
    ®∷[∣]◂⇔ .proj₂
      ( defs-ok
      , λ {_} {x = x} x∈Γ →
          ®∷◂𝟘
            (⌜ m ⌝ · 𝟘ᶜ ⟨ x ⟩  ≡⟨ PE.cong (_·_ _) $ 𝟘ᶜ-lookup x ⟩
             ⌜ m ⌝ · 𝟘         ≡⟨ ·-zeroʳ _ ⟩
             𝟘                 ∎)
      )
    where
    open Tools.Reasoning.PropositionalEquality

opaque
  unfolding Definitions-related

  -- A source substitution is related to every (matching) target
  -- substitution with respect to the (matching) zero usage context.

  ®∷[]◂𝟘ᶜ : σ ® σ′ ∷[ m ] Γ ◂ 𝟘ᶜ
  ®∷[]◂𝟘ᶜ = ®∷[∣]◂𝟘ᶜ (λ ())

opaque

  -- A variant of ▸⊩ʳ∷[]→®∷◂ (which is defined below).

  ▸⊩ʳ∷[∣]→®∷◂ :
    Definitions-related m n →
    𝟘ᶜ ▸ Δ ⊩ʳ t ∷[ m ∣ n ] A →
    t ® erase str t ∷ A ◂ ⌜ m ⌝
  ▸⊩ʳ∷[∣]→®∷◂ {m} {n} {t} {A} defs-ok =
    𝟘ᶜ ▸ Δ ⊩ʳ t ∷[ m ∣ n ] A                                             ⇔⟨ ▸⊩ʳ∷⇔ ⟩→

    (∀ {σ σ′} → ts » Δ ⊢ˢʷ σ ∷ Δ → σ ® σ′ ∷[ m ∣ n ] Δ ◂ 𝟘ᶜ →
     t [ σ ] ® erase str t T.[ σ′ ] ∷ A [ σ ] ◂ ⌜ m ⌝)                   →⟨ (λ hyp → hyp (⊢ˢʷ∷-idSubst ⊢Δ) (®∷[∣]◂𝟘ᶜ defs-ok)) ⟩

    t [ idSubst ] ® erase str t T.[ T.idSubst ] ∷ A [ idSubst ] ◂ ⌜ m ⌝  ≡⟨ PE.cong₃ (λ t v A → t ® v ∷ A ◂ _)
                                                                              (subst-id _) (TP.subst-id _) (subst-id _) ⟩→
    t ® erase str t ∷ A ◂ ⌜ m ⌝                                          □

opaque
  unfolding Definitions-related

  -- A lemma that can sometimes be used to convert the output from the
  -- fundamental lemma.

  ▸⊩ʳ∷[]→®∷◂ :
    𝟘ᶜ ▸ Δ ⊩ʳ t ∷[ m ] A →
    t ® erase str t ∷ A ◂ ⌜ m ⌝
  ▸⊩ʳ∷[]→®∷◂ = ▸⊩ʳ∷[∣]→®∷◂ (λ ())

opaque

  -- A variant of the previous lemma.

  ▸⊩ʳ∷[𝟙ᵐ]→®∷ :
    ⦃ 𝟘-well-behaved : Has-well-behaved-zero M semiring-with-meet ⦄ →
    𝟘ᶜ ▸ Δ ⊩ʳ t ∷[ 𝟙ᵐ ] A →
    t ® erase str t ∷ A
  ▸⊩ʳ∷[𝟙ᵐ]→®∷ {t} {A} =
    𝟘ᶜ ▸ Δ ⊩ʳ t ∷[ 𝟙ᵐ ] A    →⟨ ▸⊩ʳ∷[]→®∷◂ ⟩
    t ® erase str t ∷ A ◂ 𝟙  →⟨ ®∷→®∷◂ω non-trivial ⟩
    t ® erase str t ∷ A      □

------------------------------------------------------------------------
-- Some conversion lemmas

opaque
  unfolding _®_∷_

  -- Conversion for _®_∷_.

  conv-®∷ :
    ts » Δ ⊢ A ≡ B →
    t ® v ∷ A →
    t ® v ∷ B
  conv-®∷ A≡B (⊨A , t®v) =
    _ , convTermʳ ⊨A (⊢→⊨ (syntacticEq A≡B .proj₂)) A≡B t®v

opaque

  -- Conversion for _®_∷_◂_.

  conv-®∷◂ :
    ts » Δ ⊢ A ≡ B →
    t ® v ∷ A ◂ p →
    t ® v ∷ B ◂ p
  conv-®∷◂ {A} {B} {t} {v} {p} A≡B =
    t ® v ∷ A ◂ p        ⇔⟨ ®∷◂⇔ ⟩→
    (p ≢ 𝟘 → t ® v ∷ A)  →⟨ conv-®∷ A≡B ∘→_ ⟩
    (p ≢ 𝟘 → t ® v ∷ B)  ⇔˘⟨ ®∷◂⇔ ⟩→
    t ® v ∷ B ◂ p        □

opaque

  -- Conversion for _▸_⊩ʳ_∷[_∣_]_.

  conv-▸⊩ʳ∷ :
    ts » Γ ⊢ A ≡ B →
    γ ▸ Γ ⊩ʳ t ∷[ m ∣ n ] A →
    γ ▸ Γ ⊩ʳ t ∷[ m ∣ n ] B
  conv-▸⊩ʳ∷ {Γ} {A} {B} {γ} {t} {m} {n} A≡B =
    γ ▸ Γ ⊩ʳ t ∷[ m ∣ n ] A                                   ⇔⟨ ▸⊩ʳ∷⇔ ⟩→

    (∀ {σ σ′} → ts » Δ ⊢ˢʷ σ ∷ Γ → σ ® σ′ ∷[ m ∣ n ] Γ ◂ γ →
     t [ σ ] ® erase str t T.[ σ′ ] ∷ A [ σ ] ◂ ⌜ m ⌝)        →⟨ (λ hyp ⊢σ σ®σ′ →
                                                                    conv-®∷◂ (subst-⊢≡ A≡B (refl-⊢ˢʷ≡∷ ⊢σ)) $
                                                                    hyp ⊢σ σ®σ′) ⟩
    (∀ {σ σ′} → ts » Δ ⊢ˢʷ σ ∷ Γ → σ ® σ′ ∷[ m ∣ n ] Γ ◂ γ →
     t [ σ ] ® erase str t T.[ σ′ ] ∷ B [ σ ] ◂ ⌜ m ⌝)        ⇔˘⟨ ▸⊩ʳ∷⇔ ⟩→

    γ ▸ Γ ⊩ʳ t ∷[ m ∣ n ] B                                   □

------------------------------------------------------------------------
-- Closure under reduction or expansion

opaque
  unfolding _®_∷_

  -- Closure under reduction of the target language term for _®_∷_.

  ®∷-⇒* :
    vs T.⊢ v ⇒* v′ →
    t ® v ∷ A →
    t ® v′ ∷ A
  ®∷-⇒* v⇒v′ (⊨A , t®v) =
    ⊨A , targetRedSubstTerm*′ ⊨A t®v v⇒v′

opaque
  unfolding _®_∷_

  -- Closure under expansion for _®_∷_.

  ®∷-⇐* :
    t ⇛ t′ ∷ A →
    vs T.⊢ v ⇒* v′ →
    t′ ® v′ ∷ A →
    t ® v ∷ A
  ®∷-⇐* t⇒t′ v⇒v′ (⊨A , t′®v′) =
    ⊨A , redSubstTerm* ⊨A t′®v′ t⇒t′ v⇒v′

opaque
  unfolding _®_∷_◂_

  -- Closure under expansion for _®_∷_◂_.

  ®∷◂-⇐* :
    t ⇛ t′ ∷ A →
    vs T.⊢ v ⇒* v′ →
    t′ ® v′ ∷ A ◂ p →
    t ® v ∷ A ◂ p
  ®∷◂-⇐* {t} {t′} {A} {v} {v′} {p} t⇒t′ v⇒v′ =
    t′ ® v′ ∷ A ◂ p        ⇔⟨ ®∷◂⇔ ⟩→
    (p ≢ 𝟘 → t′ ® v′ ∷ A)  →⟨ ®∷-⇐* t⇒t′ v⇒v′ ∘→_ ⟩
    (p ≢ 𝟘 → t ® v ∷ A)    ⇔˘⟨ ®∷◂⇔ ⟩→
    t ® v ∷ A ◂ p          □