open import Relation.Binary.PropositionalEquality
open import nat
open import nat_add
open ≡-Reasoning

module nat_add_sym where

addSym : (n m : Nat) -> n + m ≡ m + n
addSym O       O   = refl
addSym O    (S m)  = cong S (addSym O m)
addSym (S n)   O   = cong S (addSym n O) 
addSym (S n) (S m) = {!!} -- 後述