English
If a tree t is bounded between o1 and o2, then the empty tree nil is bounded between the same bounds.
Русский
Если дерево t ограничено между o1 и o2, то пустое дерево nil ограничено теми же границами.
LaTeX
$$$\forall t\ o_1\ o_2: Bounded\ t\ o_1\ o_2 \Rightarrow Bounded\ nil\ o_1\ o_2$$$
Lean4
theorem to_nil {t : Ordnode α} : ∀ {o₁ o₂}, Bounded t o₁ o₂ → Bounded nil o₁ o₂
| none, _, _ => ⟨⟩
| some _, none, _ => ⟨⟩
| some _, some _, h => h.to_lt