have : step (n₁ + d) ≃ step n₂ := calc
  step (n₁ + d) ≃ _ := Rel.symm step_add
  step n₁ + d   ≃ _ := ‹step n₁ + d ≃ step n₂›
  step n₂       ≃ _ := Rel.refl
ℕ : Type
inst✝³ : Core ℕ
inst✝² : Induction ℕ
inst✝¹ : Addition ℕ
inst✝ : Sign ℕ
order_inst : Order ℕ
n₁ n₂ : ℕ
x✝ : step n₁ ≤ step n₂
d : ℕ
h✝ : step n₁ + d ≃ step n₂
⊢ n₁ + d ≃ n₂
ℕ : Type
inst✝³ : Core ℕ
inst✝² : Induction ℕ
inst✝¹ : Addition ℕ
inst✝ : Sign ℕ
order_inst : Order ℕ
n₁ n₂ : ℕ
x✝ : step n₁ ≤ step n₂
d : ℕ
h✝ : step n₁ + d ≃ step n₂
this : step (n₁ + d) ≃ step n₂
⊢ n₁ + d ≃ n₂