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1 open import Relation.Binary.PropositionalEquality
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2 open import nat
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3 open import nat_add
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4 open ≡-Reasoning
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5
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6 module nat_add_sym_reasoning where
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7
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8 addToRight : (n m : Nat) -> S (n + m) ≡ n + (S m)
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9 addToRight O m = refl
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10 addToRight (S n) m = cong S (addToRight n m)
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11
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12 addSym : (n m : Nat) -> n + m ≡ m + n
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13 addSym O O = refl
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14 addSym O (S m) = cong S (addSym O m)
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15 addSym (S n) O = cong S (addSym n O)
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16 addSym (S n) (S m) = begin
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17 (S n) + (S m) ≡⟨ refl ⟩
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18 S (n + S m) ≡⟨ cong S (addSym n (S m)) ⟩
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19 S ((S m) + n) ≡⟨ addToRight (S m) n ⟩
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20 S (m + S n) ≡⟨ refl ⟩
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21 (S m) + (S n) ∎
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