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Notes on lib-2.4 and a suggestion #3027

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@mechvel
  1. Data.Nat.Properties has
m∸n≤m⊔n : ∀ m n → m ∸ n ≤ m ⊔ n

Probably it is not needed because it is proved only by
≤-trans (m∸n≤m m n) (m≤m⊔n m n)

  1. Algebra.Definitions has
Identical : Op₂ A → Set _
Identical _∙_ = ∀ x y z → ((z ∙ x) ∙ (y ∙ z)) ≈ (z ∙ ((x ∙ y) ∙ z))

The name is not meaningful. Generally, many pairs of expressions are identical in algebra.

  1. Disagreement between (LeftInvertible ε x) and (x ∣ ε) in Monoid.

For example,

lemma : ∀ x → LeftInvertible _≈_ ε _∙_ x → x ∣ʳ ε
lemma x invertible-x =  invertible-x

is not type-checked.
This is one more argument for returning to the definition of
(x ∣ʳ y) as ∃[ q ] (q ∙ x ≈ y).

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