∀ {α : Type u} {β : α → Type v} {instOrd : Ord α} {t : Std.DTreeMap.Internal.Impl α β} [Std.TransOrd α]
  [inst : Inhabited α], t.WF → t.isEmpty = false → ∀ {fallback : α}, t.getKeyD t.maxKey! fallback = t.maxKey!