match this with
| Or.inl (_ : m < k) =>
  have : n < k := lt_trans ‹n < m› ‹m < k›
  exact this
| Or.inr (_ : m ≃ k) =>
  have : n < k := trans_lt_eqv_lt ‹n < m› ‹m ≃ k›
  exact this
ℕ : Type
inst✝³ : Core ℕ
inst✝² : Induction ℕ
inst✝¹ : Addition ℕ
inst✝ : Sign ℕ
order_inst : Order ℕ
n m k : ℕ
x✝¹ : n < m
x✝ : m ≤ k
this : m < k ∨ m ≃ k
⊢ n < k
no goals