calc
  step n₁ + m   ≃ _ := Addition.step_add
  step (n₁ + m) ≃ _ := AA.subst₁ (ih _ ‹n₁ ≃ n₂›)
  step (n₂ + m) ≃ _ := Rel.symm Addition.step_add
  step n₂ + m   ≃ _ := Rel.refl
ℕ : Type
inst✝² : Core ℕ
inst✝¹ : Induction ℕ
inst✝ : Addition ℕ
n₁✝ n₂✝¹ m n₁ : ℕ
ih : ∀ (y : ℕ), n₁ ≃ y → n₁ + m ≃ y + m
n₂✝ n₂ : ℕ
x✝ : step n₁ ≃ step n₂
this : n₁ ≃ n₂
⊢ step n₁ + m ≃ step n₂ + m
no goals