calc
  step (n + e) ≃ _ := Rel.symm add_step
  n + step e   ≃ _ := AA.substR (Rel.symm ‹d ≃ step e›)
  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
e : ℕ
x✝ : d ≃ step e
⊢ step (n + e) ≃ step m
no goals