∀ {α : Type u} {cmp : α → α → Ordering} {t₁ t₂ : Std.TreeSet.Raw α cmp} [Std.TransCmp cmp],
  t₁.WF → t₂.WF → ∀ {k fallback : α}, (t₁ ∩ t₂).getD k fallback = if k ∈ t₂ then t₁.getD k fallback else fallback