calc
  step n₁ + d   ≃ _ := step_add
  step (n₁ + d) ≃ _ := AA.subst₁ ‹n₁ + d ≃ 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✝ : n₁ ≤ n₂
d : ℕ
h✝ : n₁ + d ≃ n₂
⊢ step n₁ + d ≃ step n₂
no goals