------------------------------------------------------------------------
-- Some derivable properties
------------------------------------------------------------------------

{-# OPTIONS --universe-polymorphism #-}

open import Algebra

module Algebra.Props.Ring {r₁ r₂} (R : Ring r₁ r₂) where

open Ring R
import Relation.Binary.EqReasoning as EqR; open EqR setoid
open import Function
open import Data.Product

-‿*-distribˡ :  x y  - x * y  - (x * y)
-‿*-distribˡ x y = begin
  - x * y                        ≈⟨ sym $ proj₂ +-identity _ 
  - x * y + 0#                   ≈⟨ refl  +-cong  sym (proj₂ -‿inverse _) 
  - x * y + (x * y + - (x * y))  ≈⟨ sym $ +-assoc _ _ _  
  - x * y + x * y + - (x * y)    ≈⟨ sym (proj₂ distrib _ _ _)  +-cong  refl 
  (- x + x) * y + - (x * y)      ≈⟨ (proj₁ -‿inverse _  *-cong  refl)
                                       +-cong 
                                    refl 
  0# * y + - (x * y)             ≈⟨ proj₁ zero _  +-cong  refl 
  0# + - (x * y)                 ≈⟨ proj₁ +-identity _ 
  - (x * y)                      

-‿*-distribʳ :  x y  x * - y  - (x * y)
-‿*-distribʳ x y = begin
  x * - y                        ≈⟨ sym $ proj₁ +-identity _ 
  0# + x * - y                   ≈⟨ sym (proj₁ -‿inverse _)  +-cong  refl 
  - (x * y) + x * y + x * - y    ≈⟨ +-assoc _ _ _  
  - (x * y) + (x * y + x * - y)  ≈⟨ refl  +-cong  sym (proj₁ distrib _ _ _)  
  - (x * y) + x * (y + - y)      ≈⟨ refl  +-cong  (refl  *-cong  proj₂ -‿inverse _) 
  - (x * y) + x * 0#             ≈⟨ refl  +-cong  proj₂ zero _ 
  - (x * y) + 0#                 ≈⟨ proj₂ +-identity _ 
  - (x * y)