∀ {α : Type u} {β : α → Type v} {cmp : α → α → Ordering} {t : Std.DTreeMap.Raw α β cmp} [Std.TransCmp cmp]
  [inst : Std.LawfulEqCmp cmp], t.WF → ∀ {a : α} {fallback : β a}, t.isEmpty = true → t.getD a fallback = fallback