match this with
| Or.inl (_ : n < m) =>
  have : n < k := lt_trans ‹n < m› ‹m < k›
  exact this
| Or.inr (_ : n ≃ m) =>
  have : n < k := trans_eqv_lt_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 : n < m ∨ n ≃ m
⊢ n < k
no goals