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

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

open import Algebra

module Algebra.Props.AbelianGroup
{g₁ g₂} (G : AbelianGroup g₁ g₂) where

open AbelianGroup G
import Relation.Binary.EqReasoning as EqR; open EqR setoid
open import Function
open import Data.Product

private
lemma : ∀ x y → x ∙ y ∙ x ⁻¹ ≈ y
lemma x y = begin
x ∙ y ∙ x ⁻¹    ≈⟨ comm _ _ ⟨ ∙-cong ⟩ refl ⟩
y ∙ x ∙ x ⁻¹    ≈⟨ assoc _ _ _ ⟩
y ∙ (x ∙ x ⁻¹)  ≈⟨ refl ⟨ ∙-cong ⟩ proj₂ inverse _ ⟩
y ∙ ε           ≈⟨ proj₂ identity _ ⟩
y               ∎

-‿∙-comm : ∀ x y → x ⁻¹ ∙ y ⁻¹ ≈ (x ∙ y) ⁻¹
-‿∙-comm x y = begin
x ⁻¹ ∙ y ⁻¹                         ≈⟨ comm _ _ ⟩
y ⁻¹ ∙ x ⁻¹                         ≈⟨ sym \$ lem ⟨ ∙-cong ⟩ refl ⟩
x ∙ (y ∙ (x ∙ y) ⁻¹ ∙ y ⁻¹) ∙ x ⁻¹  ≈⟨ lemma _ _ ⟩
y ∙ (x ∙ y) ⁻¹ ∙ y ⁻¹               ≈⟨ lemma _ _ ⟩
(x ∙ y) ⁻¹                          ∎
where
lem = begin
x ∙ (y ∙ (x ∙ y) ⁻¹ ∙ y ⁻¹)  ≈⟨ sym \$ assoc _ _ _ ⟩
x ∙ (y ∙ (x ∙ y) ⁻¹) ∙ y ⁻¹  ≈⟨ sym \$ assoc _ _ _ ⟨ ∙-cong ⟩ refl ⟩
x ∙ y ∙ (x ∙ y) ⁻¹ ∙ y ⁻¹    ≈⟨ proj₂ inverse _ ⟨ ∙-cong ⟩ refl ⟩
ε ∙ y ⁻¹                     ≈⟨ proj₁ identity _ ⟩
y ⁻¹                         ∎
```