import Relation.Binary.PropositionalEquality as Eq
open Eq using (_≡_; refl; sym; trans)
open Eq.≡-Reasoning

open import Data.Nat using (ℕ; zero; suc; _+_)

lift : ∀ {m n : ℕ} → m ≡ n → suc m ≡ suc n
lift refl = refl

+-assoc : ∀ (m n p : ℕ) → (m + n) + p ≡ m + (n + p)
+-assoc zero n p =
  begin
    (zero + n) + p
  ≡⟨⟩
    zero + (n + p)
  ∎
+-assoc (suc m) n p =
  begin
    (suc m + n) + p
  ≡⟨⟩
    suc ((m + n) + p)
  ≡⟨ lift (+-assoc m n p) ⟩
    suc (m + (n + p))
  ≡⟨⟩
    suc m + (n + p)
  ∎

+-identity : ∀ (m : ℕ) → m + zero ≡ m
+-identity zero =
  begin
    zero + zero
  ≡⟨⟩
    zero
  ∎
+-identity (suc m) =
  begin
    suc m + zero
  ≡⟨⟩
    suc (m + zero)
  ≡⟨ lift (+-identity m) ⟩
    suc m
  ∎

+-suc : ∀ (m n : ℕ) → m + suc n ≡ suc (m + n)
+-suc zero n =
  begin
    zero + suc n
  ≡⟨⟩
    suc n
  ≡⟨⟩
    suc (zero + n)
  ∎
+-suc (suc m) n =
  begin
    suc m + suc n
  ≡⟨⟩
    suc (m + suc n)
  ≡⟨ lift (+-suc m n) ⟩
    suc (suc (m + n))
  ≡⟨⟩
    suc (suc m + n)
  ∎

+-comm : ∀ (m n : ℕ) → m + n ≡ n + m
+-comm m zero =
  begin
    m + zero
  ≡⟨ +-identity m ⟩
    m
  ≡⟨⟩
    zero + m
  ∎
+-comm m (suc n) =
  begin
    m + suc n
  ≡⟨ +-suc m n ⟩
    suc (m + n)
  ≡⟨ lift (+-comm m n) ⟩
    suc (n + m)
  ≡⟨⟩
    suc n + m
  ∎