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