calc
  n + k ≃ _ := Rel.symm ‹m ≃ n + k›
  m     ≃ _ := Rel.symm AA.identL
  0 + m ≃ _ := Rel.refl
ℕ : Type
inst✝ : Natural ℕ
n m : ℕ
neg_diff : ∀ {k : ℕ}, 0——k ≃ -1 * coe k
x✝ : n < m
k : ℕ
left✝ : Positive k
right✝ : m ≃ n + k
this : 0——k ≃ -1 * coe k
⊢ n + k ≃ 0 + m
no goals