∀ {α : Type u} {β : α → Type v} {instOrd : Ord α} {t : Std.DTreeMap.Internal.Impl α β} [Std.TransOrd α],
  t.WF → t.isEmpty = false → ∀ {fallback : α}, Std.DTreeMap.Internal.Impl.contains (t.maxKeyD fallback) t = true