∀ {α : Type u} {β : α → Type v} {cmp : α → α → Ordering} {t₁ t₂ : Std.DTreeMap.Raw α β cmp} [Std.TransCmp cmp],
  t₁.WF → t₂.WF → ∀ {k fallback : α}, k ∉ t₂ → (t₁ ∪ t₂).getKeyD k fallback = t₁.getKeyD k fallback