case zero =>
  intro (_ : n + 0 ≃ m)
  apply Or.inr
  show n ≃ m
  calc
    n     ≃ _ := Rel.symm add_zero
    n + 0 ≃ _ := ‹n + 0 ≃ 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
⊢ n + 0 ≃ m → n < m ∨ n ≃ m
no goals