∀ {α : Type u} {β : α → Type v} {cmp : α → α → Ordering} {t₁ t₂ : Std.DTreeMap.Raw α β cmp} [inst : Inhabited α]
  [Std.TransCmp cmp], t₁.WF → t₂.WF → ∀ {k : α}, k ∉ t₂ → (t₁ ∩ t₂).getKey! k = default