case step =>
  intro d (_ : n + step d ≃ m)
  apply Or.inl
  show n < m
  apply lt_step_le.mpr
  show step n ≤ m
  apply le_defn.mpr
  exists d
  show step n + d ≃ m
  calc
    step n + d   ≃ _ := step_add
    step (n + d) ≃ _ := Rel.symm add_step
    n + step d   ≃ _ := ‹n + step d ≃ m›
    m            ≃ _ := Rel.refl
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✝ : n ≤ m
d : ℕ
⊢ ∀ (m_1 : ℕ), n + step m_1 ≃ m → n < m ∨ n ≃ m
no goals