calc
  (0 + n, 0 + m) ≃ _ := AA.substL AA.identL
  (n, 0 + m)     ≃ _ := AA.substR AA.identL
  (n, m)         ≃ _ := Rel.refl
case a
ℕ : Type
inst✝ : Natural ℕ
n m : ℕ
⊢ (0 + n, 0 + m) ≃ (n, m)
no goals