have : n + (d₁ + d₂) ≃ n + 0 := calc
  n + (d₁ + d₂) ≃ _ := Rel.symm AA.assoc
  (n + d₁) + d₂ ≃ _ := AA.substL ‹n + d₁ ≃ m›
  m + d₂        ≃ _ := ‹m + d₂ ≃ n›
  n             ≃ _ := Rel.symm add_zero
  n + 0         ≃ _ := Rel.refl
ℕ : Type
inst✝³ : Core ℕ
inst✝² : Induction ℕ
inst✝¹ : Addition ℕ
inst✝ : Sign ℕ
order_inst : Order ℕ
n m : ℕ
x✝¹ : n ≤ m
x✝ : m ≤ n
d₁ : ℕ
h✝¹ : n + d₁ ≃ m
d₂ : ℕ
h✝ : m + d₂ ≃ n
⊢ n ≃ m
ℕ : Type
inst✝³ : Core ℕ
inst✝² : Induction ℕ
inst✝¹ : Addition ℕ
inst✝ : Sign ℕ
order_inst : Order ℕ
n m : ℕ
x✝¹ : n ≤ m
x✝ : m ≤ n
d₁ : ℕ
h✝¹ : n + d₁ ≃ m
d₂ : ℕ
h✝ : m + d₂ ≃ n
this : n + (d₁ + d₂) ≃ n + 0
⊢ n ≃ m