------------------------------------------------------------------------
-- The Agda standard library
--
-- Some derivable properties of Boolean algebras
------------------------------------------------------------------------

{-# OPTIONS --cubical-compatible --safe #-}

open import Algebra.Lattice.Bundles

module Algebra.Lattice.Properties.BooleanAlgebra
  {b₁ b₂} (B : BooleanAlgebra b₁ b₂)
  where

open BooleanAlgebra B

import Algebra.Lattice.Properties.DistributiveLattice as DistribLatticeProperties
open import Algebra.Core
open import Algebra.Structures _≈_
open import Algebra.Definitions _≈_
open import Algebra.Consequences.Setoid setoid
open import Algebra.Bundles
open import Algebra.Lattice.Structures _≈_
open import Relation.Binary.Reasoning.Setoid setoid
open import Function.Base using (id; _$_; _⟨_⟩_)
open import Function.Bundles using (_⇔_; module Equivalence)
open import Data.Product.Base using (_,_)

------------------------------------------------------------------------
-- Export properties from distributive lattices

open DistribLatticeProperties distributiveLattice public

------------------------------------------------------------------------
-- The dual construction is also a boolean algebra

∧-∨-isBooleanAlgebra : IsBooleanAlgebra _∧_ _∨_ ¬_  
∧-∨-isBooleanAlgebra = record
  { isDistributiveLattice = ∧-∨-isDistributiveLattice
  ; ∨-complement          = ∧-complement
  ; ∧-complement          = ∨-complement
  ; ¬-cong                = ¬-cong
  }

∧-∨-booleanAlgebra : BooleanAlgebra _ _
∧-∨-booleanAlgebra = record
  { isBooleanAlgebra = ∧-∨-isBooleanAlgebra
  }

------------------------------------------------------------------------
-- (∨, ∧, ⊥, ⊤) and (∧, ∨, ⊤, ⊥) are commutative semirings

∧-identityʳ : RightIdentity  _∧_
∧-identityʳ x = begin
  x            ≈⟨ ∧-congˡ (sym (∨-complementʳ _)) 
  x  (x  ¬ x)  ≈⟨ ∧-absorbs-∨ _ _ 
  x              

∧-identityˡ : LeftIdentity  _∧_
∧-identityˡ = comm∧idʳ⇒idˡ ∧-comm ∧-identityʳ

∧-identity : Identity  _∧_
∧-identity = ∧-identityˡ , ∧-identityʳ

∨-identityʳ : RightIdentity  _∨_
∨-identityʳ x = begin
  x            ≈⟨ ∨-congˡ $ sym (∧-complementʳ _) 
  x  x  ¬ x    ≈⟨ ∨-absorbs-∧ _ _ 
  x              

∨-identityˡ : LeftIdentity  _∨_
∨-identityˡ = comm∧idʳ⇒idˡ ∨-comm ∨-identityʳ

∨-identity : Identity  _∨_
∨-identity = ∨-identityˡ , ∨-identityʳ

∧-zeroʳ : RightZero  _∧_
∧-zeroʳ x = begin
  x            ≈⟨ ∧-congˡ (∧-complementʳ x) 
  x   x   ¬ x  ≈⟨ ∧-assoc x x (¬ x) 
  (x  x)  ¬ x  ≈⟨  ∧-congʳ (∧-idem x) 
  x        ¬ x  ≈⟨  ∧-complementʳ x 
                

∧-zeroˡ : LeftZero  _∧_
∧-zeroˡ = comm∧zeʳ⇒zeˡ ∧-comm ∧-zeroʳ

∧-zero : Zero  _∧_
∧-zero = ∧-zeroˡ , ∧-zeroʳ

∨-zeroʳ :  x  x    
∨-zeroʳ x = begin
  x            ≈⟨ ∨-congˡ (∨-complementʳ x) 
  x   x   ¬ x  ≈⟨ ∨-assoc x x (¬ x) 
  (x  x)  ¬ x  ≈⟨ ∨-congʳ (∨-idem x) 
  x        ¬ x  ≈⟨ ∨-complementʳ x 
                

∨-zeroˡ : LeftZero  _∨_
∨-zeroˡ = comm∧zeʳ⇒zeˡ ∨-comm ∨-zeroʳ

∨-zero : Zero  _∨_
∨-zero = ∨-zeroˡ , ∨-zeroʳ

∨-⊥-isMonoid : IsMonoid _∨_ 
∨-⊥-isMonoid = record
  { isSemigroup = ∨-isSemigroup
  ; identity    = ∨-identity
  }

∧-⊤-isMonoid : IsMonoid _∧_ 
∧-⊤-isMonoid = record
  { isSemigroup = ∧-isSemigroup
  ; identity    = ∧-identity
  }

∨-⊥-isCommutativeMonoid : IsCommutativeMonoid _∨_ 
∨-⊥-isCommutativeMonoid = record
  { isMonoid = ∨-⊥-isMonoid
  ; comm     = ∨-comm
  }

∧-⊤-isCommutativeMonoid : IsCommutativeMonoid _∧_ 
∧-⊤-isCommutativeMonoid = record
  { isMonoid = ∧-⊤-isMonoid
  ; comm     = ∧-comm
  }

∨-∧-isSemiring : IsSemiring _∨_ _∧_  
∨-∧-isSemiring = record
  { isSemiringWithoutAnnihilatingZero = record
    { +-isCommutativeMonoid = ∨-⊥-isCommutativeMonoid
    ; *-cong = ∧-cong
    ; *-assoc = ∧-assoc
    ; *-identity = ∧-identity
    ; distrib = ∧-distrib-∨
    }
  ; zero = ∧-zero
  }

∧-∨-isSemiring : IsSemiring _∧_ _∨_  
∧-∨-isSemiring = record
  { isSemiringWithoutAnnihilatingZero = record
    { +-isCommutativeMonoid = ∧-⊤-isCommutativeMonoid
    ; *-cong = ∨-cong
    ; *-assoc = ∨-assoc
    ; *-identity = ∨-identity
    ; distrib = ∨-distrib-∧
    }
  ; zero = ∨-zero
  }

∨-∧-isCommutativeSemiring : IsCommutativeSemiring _∨_ _∧_  
∨-∧-isCommutativeSemiring = record
  { isSemiring = ∨-∧-isSemiring
  ; *-comm = ∧-comm
  }

∧-∨-isCommutativeSemiring : IsCommutativeSemiring _∧_ _∨_  
∧-∨-isCommutativeSemiring = record
  { isSemiring = ∧-∨-isSemiring
  ; *-comm = ∨-comm
  }

∨-∧-commutativeSemiring : CommutativeSemiring _ _
∨-∧-commutativeSemiring = record
  { isCommutativeSemiring = ∨-∧-isCommutativeSemiring
  }

∧-∨-commutativeSemiring : CommutativeSemiring _ _
∧-∨-commutativeSemiring = record
  { isCommutativeSemiring = ∧-∨-isCommutativeSemiring
  }

------------------------------------------------------------------------
-- Some other properties

-- I took the statement of this lemma (called Uniqueness of
-- Complements) from some course notes, "Boolean Algebra", written
-- by Gert Smolka.

private
  lemma :  x y  x  y    x  y    ¬ x  y
  lemma x y x∧y=⊥ x∨y=⊤ = begin
    ¬ x                ≈⟨ ∧-identityʳ _ 
    ¬ x              ≈⟨ ∧-congˡ x∨y=⊤ 
    ¬ x  (x  y)      ≈⟨  ∧-distribˡ-∨ _ _ _ 
    ¬ x  x  ¬ x  y  ≈⟨  ∨-congʳ $ ∧-complementˡ _ 
      ¬ x  y        ≈⟨ ∨-congʳ x∧y=⊥ 
    x  y  ¬ x  y    ≈⟨ ∧-distribʳ-∨ _ _ _ 
    (x  ¬ x)  y      ≈⟨  ∧-congʳ $ ∨-complementʳ _ 
      y              ≈⟨  ∧-identityˡ _ 
    y                  

⊥≉⊤ : ¬   
⊥≉⊤ = lemma   (∧-identityʳ _) (∨-zeroʳ _)

⊤≉⊥ : ¬   
⊤≉⊥ = lemma   (∧-zeroʳ _) (∨-identityʳ _)

¬-involutive : Involutive ¬_
¬-involutive x = lemma (¬ x) x (∧-complementˡ _) (∨-complementˡ _)

deMorgan₁ :  x y  ¬ (x  y)  ¬ x  ¬ y
deMorgan₁ x y = lemma (x  y) (¬ x  ¬ y) lem₁ lem₂
  where
  lem₁ = begin
    (x  y)  (¬ x  ¬ y)          ≈⟨ ∧-distribˡ-∨ _ _ _ 
    (x  y)  ¬ x  (x  y)  ¬ y  ≈⟨ ∨-congʳ $ ∧-congʳ $ ∧-comm _ _ 
    (y  x)  ¬ x  (x  y)  ¬ y  ≈⟨ ∧-assoc _ _ _  ∨-cong  ∧-assoc _ _ _ 
    y  (x  ¬ x)  x  (y  ¬ y)  ≈⟨ (∧-congˡ $ ∧-complementʳ _)  ∨-cong 
                                      (∧-congˡ $ ∧-complementʳ _) 
    (y  )  (x  )              ≈⟨ ∧-zeroʳ _  ∨-cong  ∧-zeroʳ _ 
                                ≈⟨ ∨-identityʳ _ 
                                  

  lem₃ = begin
    (x  y)  ¬ x          ≈⟨ ∨-distribʳ-∧ _ _ _ 
    (x  ¬ x)  (y  ¬ x)  ≈⟨ ∧-congʳ $ ∨-complementʳ _ 
      (y  ¬ x)          ≈⟨ ∧-identityˡ _ 
    y  ¬ x                ≈⟨ ∨-comm _ _ 
    ¬ x  y                

  lem₂ = begin
    (x  y)  (¬ x  ¬ y)  ≈⟨ ∨-assoc _ _ _ 
    ((x  y)  ¬ x)  ¬ y  ≈⟨ ∨-congʳ lem₃ 
    (¬ x  y)  ¬ y        ≈⟨ ∨-assoc _ _ _ 
    ¬ x  (y  ¬ y)        ≈⟨ ∨-congˡ $ ∨-complementʳ _ 
    ¬ x                  ≈⟨ ∨-zeroʳ _ 
                          

deMorgan₂ :  x y  ¬ (x  y)  ¬ x  ¬ y
deMorgan₂ x y = begin
  ¬ (x  y)          ≈⟨ ¬-cong $ ((¬-involutive _)  ∨-cong  (¬-involutive _)) 
  ¬ (¬ ¬ x  ¬ ¬ y)  ≈⟨ ¬-cong $ deMorgan₁ _ _ 
  ¬ ¬ (¬ x  ¬ y)    ≈⟨ ¬-involutive _ 
  ¬ x  ¬ y          

------------------------------------------------------------------------
-- (⊕, ∧, id, ⊥, ⊤) is a commutative ring

-- This construction is parameterised over the definition of xor.

module XorRing
  (xor : Op₂ Carrier)
  (⊕-def :  x y  xor x y  (x  y)  ¬ (x  y))
  where

  private
    infixl 6 _⊕_

    _⊕_ : Op₂ Carrier
    _⊕_ = xor

    helper :  {x y u v}  x  y  u  v  x  ¬ u  y  ¬ v
    helper x≈y u≈v = x≈y  ∧-cong  ¬-cong u≈v

  ⊕-cong : Congruent₂ _⊕_
  ⊕-cong {x} {y} {u} {v} x≈y u≈v = begin
    x  u                ≈⟨  ⊕-def _ _ 
    (x  u)  ¬ (x  u)  ≈⟨  helper (x≈y  ∨-cong  u≈v)
                                    (x≈y  ∧-cong  u≈v) 
    (y  v)  ¬ (y  v)  ≈⟨ ⊕-def _ _ 
    y  v                

  ⊕-comm : Commutative _⊕_
  ⊕-comm x y = begin
    x  y                ≈⟨  ⊕-def _ _ 
    (x  y)  ¬ (x  y)  ≈⟨  helper (∨-comm _ _) (∧-comm _ _) 
    (y  x)  ¬ (y  x)  ≈⟨ ⊕-def _ _ 
    y  x                

  ¬-distribˡ-⊕ :  x y  ¬ (x  y)  ¬ x  y
  ¬-distribˡ-⊕ x y = begin
    ¬ (x  y)                              ≈⟨ ¬-cong $ ⊕-def _ _ 
    ¬ ((x  y)  (¬ (x  y)))              ≈⟨ ¬-cong (∧-distribʳ-∨ _ _ _) 
    ¬ ((x  ¬ (x  y))  (y  ¬ (x  y)))  ≈⟨ ¬-cong $ ∨-congˡ $ ∧-congˡ $ ¬-cong (∧-comm _ _) 
    ¬ ((x  ¬ (x  y))  (y  ¬ (y  x)))  ≈⟨ ¬-cong $ lem _ _  ∨-cong  lem _ _ 
    ¬ ((x  ¬ y)  (y  ¬ x))              ≈⟨ deMorgan₂ _ _ 
    ¬ (x  ¬ y)  ¬ (y  ¬ x)              ≈⟨ ∧-congʳ $ deMorgan₁ _ _ 
    (¬ x  (¬ ¬ y))  ¬ (y  ¬ x)          ≈⟨ helper (∨-congˡ $ ¬-involutive _) (∧-comm _ _) 
    (¬ x  y)  ¬ (¬ x  y)                ≈⟨ ⊕-def _ _ 
    ¬ x  y                                
    where
    lem :  x y  x  ¬ (x  y)  x  ¬ y
    lem x y = begin
      x  ¬ (x  y)          ≈⟨ ∧-congˡ $ deMorgan₁ _ _ 
      x  (¬ x  ¬ y)        ≈⟨ ∧-distribˡ-∨ _ _ _ 
      (x  ¬ x)  (x  ¬ y)  ≈⟨ ∨-congʳ $ ∧-complementʳ _ 
        (x  ¬ y)          ≈⟨ ∨-identityˡ _ 
      x  ¬ y                

  ¬-distribʳ-⊕ :  x y  ¬ (x  y)  x  ¬ y
  ¬-distribʳ-⊕ x y = begin
    ¬ (x  y)  ≈⟨ ¬-cong $ ⊕-comm _ _ 
    ¬ (y  x)  ≈⟨ ¬-distribˡ-⊕ _ _ 
    ¬ y  x    ≈⟨ ⊕-comm _ _ 
    x  ¬ y    

  ⊕-annihilates-¬ :  x y  x  y  ¬ x  ¬ y
  ⊕-annihilates-¬ x y = begin
    x  y        ≈⟨ ¬-involutive _ 
    ¬ ¬ (x  y)  ≈⟨  ¬-cong $ ¬-distribˡ-⊕ _ _ 
    ¬ (¬ x  y)  ≈⟨  ¬-distribʳ-⊕ _ _ 
    ¬ x  ¬ y    

  ⊕-identityˡ : LeftIdentity  _⊕_
  ⊕-identityˡ x = begin
      x                ≈⟨ ⊕-def _ _ 
    (  x)  ¬ (  x)  ≈⟨ helper (∨-identityˡ _) (∧-zeroˡ _) 
    x  ¬               ≈⟨ ∧-congˡ ⊥≉⊤ 
    x                  ≈⟨ ∧-identityʳ _ 
    x                    

  ⊕-identityʳ : RightIdentity  _⊕_
  ⊕-identityʳ _ = ⊕-comm _ _  trans  ⊕-identityˡ _

  ⊕-identity : Identity  _⊕_
  ⊕-identity = ⊕-identityˡ , ⊕-identityʳ

  ⊕-inverseˡ : LeftInverse  id _⊕_
  ⊕-inverseˡ x = begin
    x  x               ≈⟨ ⊕-def _ _ 
    (x  x)  ¬ (x  x) ≈⟨ helper (∨-idem _) (∧-idem _) 
    x  ¬ x             ≈⟨ ∧-complementʳ _ 
                       

  ⊕-inverseʳ : RightInverse  id _⊕_
  ⊕-inverseʳ _ = ⊕-comm _ _  trans  ⊕-inverseˡ _

  ⊕-inverse : Inverse  id _⊕_
  ⊕-inverse = ⊕-inverseˡ , ⊕-inverseʳ

  ∧-distribˡ-⊕ : _∧_ DistributesOverˡ _⊕_
  ∧-distribˡ-⊕ x y z = begin
    x  (y  z)                ≈⟨ ∧-congˡ $ ⊕-def _ _ 
    x  ((y  z)  ¬ (y  z))  ≈⟨ ∧-assoc _ _ _ 
    (x  (y  z))  ¬ (y  z)  ≈⟨ ∧-congˡ $ deMorgan₁ _ _ 
    (x  (y  z)) 
    (¬ y  ¬ z)                ≈⟨ ∨-identityˡ _ 
     
    ((x  (y  z)) 
    (¬ y  ¬ z))               ≈⟨ ∨-congʳ lem₃ 
    ((x  (y  z))  ¬ x) 
    ((x  (y  z)) 
    (¬ y  ¬ z))               ≈⟨ ∧-distribˡ-∨ _ _ _ 
    (x  (y  z)) 
    (¬ x  (¬ y  ¬ z))        ≈⟨ ∧-congˡ $ ∨-congˡ (deMorgan₁ _ _) 
    (x  (y  z)) 
    (¬ x  ¬ (y  z))          ≈⟨ ∧-congˡ (deMorgan₁ _ _) 
    (x  (y  z)) 
    ¬ (x  (y  z))            ≈⟨ helper refl lem₁ 
    (x  (y  z)) 
    ¬ ((x  y)  (x  z))      ≈⟨ ∧-congʳ $ ∧-distribˡ-∨ _ _ _ 
    ((x  y)  (x  z)) 
    ¬ ((x  y)  (x  z))      ≈⟨ ⊕-def _ _ 
    (x  y)  (x  z)          
      where
      lem₂ = begin
        x  (y  z)  ≈⟨ ∧-assoc _ _ _ 
        (x  y)  z  ≈⟨ ∧-congʳ $ ∧-comm _ _ 
        (y  x)  z  ≈⟨ ∧-assoc _ _ _ 
        y  (x  z)  

      lem₁ = begin
        x  (y  z)        ≈⟨ ∧-congʳ (∧-idem _) 
        (x  x)  (y  z)  ≈⟨ ∧-assoc _ _ _ 
        x  (x  (y  z))  ≈⟨ ∧-congˡ lem₂ 
        x  (y  (x  z))  ≈⟨ ∧-assoc _ _ _ 
        (x  y)  (x  z)  

      lem₃ = begin
                              ≈⟨ ∧-zeroʳ _ 
        (y  z)              ≈⟨ ∧-congˡ (∧-complementʳ _) 
        (y  z)  (x  ¬ x)    ≈⟨ ∧-assoc _ _ _ 
        ((y  z)  x)  ¬ x    ≈⟨ ∧-congʳ (∧-comm _ _) 
        (x  (y  z))  ¬ x    

  ∧-distribʳ-⊕ : _∧_ DistributesOverʳ _⊕_
  ∧-distribʳ-⊕ = comm∧distrˡ⇒distrʳ ⊕-cong ∧-comm ∧-distribˡ-⊕

  ∧-distrib-⊕ : _∧_ DistributesOver _⊕_
  ∧-distrib-⊕ = ∧-distribˡ-⊕ , ∧-distribʳ-⊕

  private

    lemma₂ :  x y u v 
             (x  y)  (u  v) 
             ((x  u)  (y  u)) 
             ((x  v)  (y  v))
    lemma₂ x y u v = begin
        (x  y)  (u  v)              ≈⟨ ∨-distribˡ-∧ _ _ _ 
        ((x  y)  u)  ((x  y)  v)  ≈⟨ ∨-distribʳ-∧ _ _ _
                                             ∧-cong 
                                          ∨-distribʳ-∧ _ _ _ 
        ((x  u)  (y  u)) 
        ((x  v)  (y  v))            

  ⊕-assoc : Associative _⊕_
  ⊕-assoc x y z = sym $ begin
    x  (y  z)                                ≈⟨ ⊕-cong refl (⊕-def _ _) 
    x  ((y  z)  ¬ (y  z))                  ≈⟨ ⊕-def _ _ 
      (x  ((y  z)  ¬ (y  z))) 
    ¬ (x  ((y  z)  ¬ (y  z)))              ≈⟨ ∧-cong lem₃ lem₄ 
    (((x  y)  z)  ((x  ¬ y)  ¬ z)) 
    (((¬ x  ¬ y)  z)  ((¬ x  y)  ¬ z))    ≈⟨ ∧-assoc _ _ _ 
    ((x  y)  z) 
    (((x  ¬ y)  ¬ z) 
     (((¬ x  ¬ y)  z)  ((¬ x  y)  ¬ z)))  ≈⟨ ∧-congˡ lem₅ 
    ((x  y)  z) 
    (((¬ x  ¬ y)  z) 
     (((x  ¬ y)  ¬ z)  ((¬ x  y)  ¬ z)))  ≈⟨ ∧-assoc _ _ _ 
    (((x  y)  z)  ((¬ x  ¬ y)  z)) 
    (((x  ¬ y)  ¬ z)  ((¬ x  y)  ¬ z))    ≈⟨ ∧-cong lem₁ lem₂ 
      (((x  y)  ¬ (x  y))  z) 
    ¬ (((x  y)  ¬ (x  y))  z)              ≈⟨ ⊕-def _ _ 
    ((x  y)  ¬ (x  y))  z                  ≈⟨ ⊕-cong (⊕-def _ _) refl 
    (x  y)  z                                
    where
    lem₁ = begin
      ((x  y)  z)  ((¬ x  ¬ y)  z)  ≈⟨ ∨-distribʳ-∧ _ _ _ 
      ((x  y)  (¬ x  ¬ y))  z        ≈⟨ ∨-congʳ $ ∧-congˡ (deMorgan₁ _ _) 
      ((x  y)  ¬ (x  y))  z          

    lem₂′ = begin
      (x  ¬ y)  (¬ x  y)              ≈⟨ ∧-cong (∧-identityˡ _) (∧-identityʳ _) 
      (  (x  ¬ y))  ((¬ x  y)  )  ≈⟨ ∧-cong
                                              (∧-cong (∨-complementˡ _) (∨-comm _ _))
                                              (∧-congˡ $ ∨-complementˡ _) 
      ((¬ x  x)  (¬ y  x)) 
      ((¬ x  y)  (¬ y  y))            ≈⟨ lemma₂ _ _ _ _ 
      (¬ x  ¬ y)  (x  y)              ≈⟨ ∨-cong (deMorgan₂ _ _) (¬-involutive _) 
      ¬ (x  y)  ¬ ¬ (x  y)            ≈⟨ deMorgan₁ _ _ 
      ¬ ((x  y)  ¬ (x  y))            

    lem₂ = begin
      ((x  ¬ y)  ¬ z)  ((¬ x  y)  ¬ z)  ≈⟨ ∨-distribʳ-∧ _ _ _ 
      ((x  ¬ y)  (¬ x  y))  ¬ z          ≈⟨ ∨-congʳ lem₂′ 
      ¬ ((x  y)  ¬ (x  y))  ¬ z          ≈⟨ deMorgan₁ _ _ 
      ¬ (((x  y)  ¬ (x  y))  z)          

    lem₃ = begin
      x  ((y  z)  ¬ (y  z))          ≈⟨ ∨-congˡ $ ∧-congˡ $ deMorgan₁ _ _ 
      x  ((y  z)  (¬ y  ¬ z))        ≈⟨ ∨-distribˡ-∧ _ _ _ 
      (x  (y  z))  (x  (¬ y  ¬ z))  ≈⟨ ∨-assoc _ _ _  ∧-cong  ∨-assoc _ _ _ 
      ((x  y)  z)  ((x  ¬ y)  ¬ z)  

    lem₄′ = begin
      ¬ ((y  z)  ¬ (y  z))    ≈⟨ deMorgan₁ _ _ 
      ¬ (y  z)  ¬ ¬ (y  z)    ≈⟨ deMorgan₂ _ _  ∨-cong  ¬-involutive _ 
      (¬ y  ¬ z)  (y  z)      ≈⟨ lemma₂ _ _ _ _ 
      ((¬ y  y)  (¬ z  y)) 
      ((¬ y  z)  (¬ z  z))    ≈⟨ (∨-complementˡ _  ∧-cong  ∨-comm _ _)
                                       ∧-cong 
                                   (∧-congˡ $ ∨-complementˡ _) 
      (  (y  ¬ z)) 
      ((¬ y  z)  )            ≈⟨ ∧-identityˡ _  ∧-cong  ∧-identityʳ _ 
      (y  ¬ z)  (¬ y  z)      

    lem₄ = begin
      ¬ (x  ((y  z)  ¬ (y  z)))  ≈⟨ deMorgan₁ _ _ 
      ¬ x  ¬ ((y  z)  ¬ (y  z))  ≈⟨ ∨-congˡ lem₄′ 
      ¬ x  ((y  ¬ z)  (¬ y  z))  ≈⟨ ∨-distribˡ-∧ _ _ _ 
      (¬ x  (y      ¬ z)) 
      (¬ x  (¬ y  z))              ≈⟨ ∨-assoc _ _ _  ∧-cong  ∨-assoc _ _ _ 
      ((¬ x  y)      ¬ z) 
      ((¬ x  ¬ y)  z)              ≈⟨ ∧-comm _ _ 
      ((¬ x  ¬ y)  z) 
      ((¬ x  y)      ¬ z)          

    lem₅ = begin
      ((x  ¬ y)  ¬ z) 
      (((¬ x  ¬ y)  z)  ((¬ x  y)  ¬ z))    ≈⟨ ∧-assoc _ _ _ 
      (((x  ¬ y)  ¬ z)  ((¬ x  ¬ y)  z)) 
      ((¬ x  y)  ¬ z)                          ≈⟨ ∧-congʳ $ ∧-comm _ _ 
      (((¬ x  ¬ y)  z)  ((x  ¬ y)  ¬ z)) 
      ((¬ x  y)  ¬ z)                          ≈⟨ ∧-assoc _ _ _ 
      ((¬ x  ¬ y)  z) 
      (((x  ¬ y)  ¬ z)  ((¬ x  y)  ¬ z))    

  ⊕-isMagma : IsMagma _⊕_
  ⊕-isMagma = record
    { isEquivalence = isEquivalence
    ; ∙-cong        = ⊕-cong
    }

  ⊕-isSemigroup : IsSemigroup _⊕_
  ⊕-isSemigroup = record
    { isMagma = ⊕-isMagma
    ; assoc   = ⊕-assoc
    }

  ⊕-⊥-isMonoid : IsMonoid _⊕_ 
  ⊕-⊥-isMonoid = record
    { isSemigroup = ⊕-isSemigroup
    ; identity    = ⊕-identity
    }

  ⊕-⊥-isGroup : IsGroup _⊕_  id
  ⊕-⊥-isGroup = record
    { isMonoid = ⊕-⊥-isMonoid
    ; inverse  = ⊕-inverse
    ; ⁻¹-cong  = id
    }

  ⊕-⊥-isAbelianGroup : IsAbelianGroup _⊕_  id
  ⊕-⊥-isAbelianGroup = record
    { isGroup = ⊕-⊥-isGroup
    ; comm    = ⊕-comm
    }

  ⊕-∧-isRing : IsRing _⊕_ _∧_ id  
  ⊕-∧-isRing = record
    { +-isAbelianGroup = ⊕-⊥-isAbelianGroup
    ; *-cong = ∧-cong
    ; *-assoc = ∧-assoc
    ; *-identity = ∧-identity
    ; distrib = ∧-distrib-⊕
    }

  ⊕-∧-isCommutativeRing : IsCommutativeRing _⊕_ _∧_ id  
  ⊕-∧-isCommutativeRing = record
    { isRing = ⊕-∧-isRing
    ; *-comm = ∧-comm
    }

  ⊕-∧-commutativeRing : CommutativeRing _ _
  ⊕-∧-commutativeRing = record
    { isCommutativeRing = ⊕-∧-isCommutativeRing
    }


infixl 6 _⊕_

_⊕_ : Op₂ Carrier
x  y = (x  y)  ¬ (x  y)

module DefaultXorRing = XorRing _⊕_  _ _  refl)