calc
  n      ≃ _ := Rel.symm add_zero
  n + 0  ≃ _ := AA.substR (Rel.symm ‹d ≃ 0›)
  n + d  ≃ _ := ‹n + d ≃ step m›
  step m ≃ _ := Rel.refl
case x.h
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 ≤ step m
d : ℕ
h✝ : n + d ≃ step m
motive : ℕ → Prop := fun x => d ≃ x → n ≤ m ∨ n ≃ step m
x✝ : d ≃ 0
⊢ n ≃ step m
no goals