```------------------------------------------------------------------------
-- The Agda standard library
--
-- Unary relations
------------------------------------------------------------------------

module Relation.Unary where

open import Data.Empty
open import Function
open import Data.Unit.Base using (⊤)
open import Data.Product
open import Data.Sum
open import Level
open import Relation.Nullary
open import Relation.Binary.Core using (_≡_)

------------------------------------------------------------------------
-- Unary relations

Pred : ∀ {a} → Set a → (ℓ : Level) → Set (a ⊔ suc ℓ)
Pred A ℓ = A → Set ℓ

------------------------------------------------------------------------
-- Unary relations can be seen as sets

-- I.e., they can be seen as subsets of the universe of discourse.

module _ {a} {A : Set a} -- The universe of discourse.
where

-- Set membership.

infix 4 _∈_ _∉_

_∈_ : ∀ {ℓ} → A → Pred A ℓ → Set _
x ∈ P = P x

_∉_ : ∀ {ℓ} → A → Pred A ℓ → Set _
x ∉ P = ¬ x ∈ P

-- The empty set.

∅ : Pred A zero
∅ = λ _ → ⊥

-- The property of being empty.

Empty : ∀ {ℓ} → Pred A ℓ → Set _
Empty P = ∀ x → x ∉ P

∅-Empty : Empty ∅
∅-Empty x ()

-- The singleton set.
｛_｝ : A → Pred A a
｛ x ｝ = _≡_ x

-- The universe, i.e. the subset containing all elements in A.

U : Pred A zero
U = λ _ → ⊤

-- The property of being universal.

Universal : ∀ {ℓ} → Pred A ℓ → Set _
Universal P = ∀ x → x ∈ P

U-Universal : Universal U
U-Universal = λ _ → _

-- Set complement.

∁ : ∀ {ℓ} → Pred A ℓ → Pred A ℓ
∁ P = λ x → x ∉ P

∁∅-Universal : Universal (∁ ∅)
∁∅-Universal = λ x x∈∅ → x∈∅

∁U-Empty : Empty (∁ U)
∁U-Empty = λ x x∈∁U → x∈∁U _

-- P ⊆ Q means that P is a subset of Q. _⊆′_ is a variant of _⊆_.

infix 4 _⊆_ _⊇_ _⊆′_ _⊇′_

_⊆_ : ∀ {ℓ₁ ℓ₂} → Pred A ℓ₁ → Pred A ℓ₂ → Set _
P ⊆ Q = ∀ {x} → x ∈ P → x ∈ Q

_⊆′_ : ∀ {ℓ₁ ℓ₂} → Pred A ℓ₁ → Pred A ℓ₂ → Set _
P ⊆′ Q = ∀ x → x ∈ P → x ∈ Q

_⊇_ : ∀ {ℓ₁ ℓ₂} → Pred A ℓ₁ → Pred A ℓ₂ → Set _
Q ⊇ P = P ⊆ Q

_⊇′_ : ∀ {ℓ₁ ℓ₂} → Pred A ℓ₁ → Pred A ℓ₂ → Set _
Q ⊇′ P = P ⊆′ Q

∅-⊆ : ∀ {ℓ} → (P : Pred A ℓ) → ∅ ⊆ P
∅-⊆ P ()

⊆-U : ∀ {ℓ} → (P : Pred A ℓ) → P ⊆ U
⊆-U P _ = _

-- Positive version of non-disjointness, dual to inclusion.

infix 4 _≬_

_≬_ : ∀ {ℓ₁ ℓ₂} → Pred A ℓ₁ → Pred A ℓ₂ → Set _
P ≬ Q = ∃ λ x → x ∈ P × x ∈ Q

-- Set union.

infixr 6 _∪_

_∪_ : ∀ {ℓ₁ ℓ₂} → Pred A ℓ₁ → Pred A ℓ₂ → Pred A _
P ∪ Q = λ x → x ∈ P ⊎ x ∈ Q

-- Set intersection.

infixr 7 _∩_

_∩_ : ∀ {ℓ₁ ℓ₂} → Pred A ℓ₁ → Pred A ℓ₂ → Pred A _
P ∩ Q = λ x → x ∈ P × x ∈ Q

-- Implication.

infixl 8 _⇒_

_⇒_ : ∀ {ℓ₁ ℓ₂} → Pred A ℓ₁ → Pred A ℓ₂ → Pred A _
P ⇒ Q = λ x → x ∈ P → x ∈ Q

-- Infinitary union and intersection.

infix 9 ⋃ ⋂

⋃ : ∀ {ℓ i} (I : Set i) → (I → Pred A ℓ) → Pred A _
⋃ I P = λ x → Σ[ i ∈ I ] P i x

syntax ⋃ I (λ i → P) = ⋃[ i ∶ I ] P

⋂ : ∀ {ℓ i} (I : Set i) → (I → Pred A ℓ) → Pred A _
⋂ I P = λ x → (i : I) → P i x

syntax ⋂ I (λ i → P) = ⋂[ i ∶ I ] P

------------------------------------------------------------------------
-- Unary relation combinators

infixr  2 _⟨×⟩_
infixr  2 _⟨⊙⟩_
infixr  1 _⟨⊎⟩_
infixr  0 _⟨→⟩_
infixl  9 _⟨·⟩_
infix  10 _~
infixr  9 _⟨∘⟩_
infixr  2 _//_ _\\_

_⟨×⟩_ : ∀ {a b ℓ₁ ℓ₂} {A : Set a} {B : Set b} →
Pred A ℓ₁ → Pred B ℓ₂ → Pred (A × B) _
(P ⟨×⟩ Q) (x , y) = x ∈ P × y ∈ Q

_⟨⊙⟩_ : ∀ {a b ℓ₁ ℓ₂} {A : Set a} {B : Set b} →
Pred A ℓ₁ → Pred B ℓ₂ → Pred (A × B) _
(P ⟨⊙⟩ Q) (x , y) = x ∈ P ⊎ y ∈ Q

_⟨⊎⟩_ : ∀ {a b ℓ} {A : Set a} {B : Set b} →
Pred A ℓ → Pred B ℓ → Pred (A ⊎ B) _
P ⟨⊎⟩ Q = [ P , Q ]

_⟨→⟩_ : ∀ {a b ℓ₁ ℓ₂} {A : Set a} {B : Set b} →
Pred A ℓ₁ → Pred B ℓ₂ → Pred (A → B) _
(P ⟨→⟩ Q) f = P ⊆ Q ∘ f

_⟨·⟩_ : ∀ {a b ℓ₁ ℓ₂} {A : Set a} {B : Set b}
(P : Pred A ℓ₁) (Q : Pred B ℓ₂) →
(P ⟨×⟩ (P ⟨→⟩ Q)) ⊆ Q ∘ uncurry (flip _\$_)
(P ⟨·⟩ Q) (p , f) = f p

-- Converse.

_~ : ∀ {a b ℓ} {A : Set a} {B : Set b} →
Pred (A × B) ℓ → Pred (B × A) ℓ
P ~ = P ∘ swap

-- Composition.

_⟨∘⟩_ : ∀ {a b c ℓ₁ ℓ₂} {A : Set a} {B : Set b} {C : Set c} →
Pred (A × B) ℓ₁ → Pred (B × C) ℓ₂ → Pred (A × C) _
(P ⟨∘⟩ Q) (x , z) = ∃ λ y → (x , y) ∈ P × (y , z) ∈ Q

-- Post and pre-division.

_//_ : ∀ {a b c ℓ₁ ℓ₂} {A : Set a} {B : Set b} {C : Set c} →
Pred (A × C) ℓ₁ → Pred (B × C) ℓ₂ → Pred (A × B) _
(P // Q) (x , y) = Q ∘ _,_ y ⊆ P ∘ _,_ x

_\\_ : ∀ {a b c ℓ₁ ℓ₂} {A : Set a} {B : Set b} {C : Set c} →
Pred (A × C) ℓ₁ → Pred (A × B) ℓ₂ → Pred (B × C) _
P \\ Q = (P ~ // Q ~) ~

------------------------------------------------------------------------
-- Properties of unary relations

Decidable : ∀ {a ℓ} {A : Set a} (P : Pred A ℓ) → Set _
Decidable P = ∀ x → Dec (P x)
```