∀ {α : Type u} {β : α → Type v} {instOrd : Ord α} {t : Std.DTreeMap.Internal.Impl α β} [Std.TransOrd α],
  t.WF → ∀ {k : α}, (Std.DTreeMap.Internal.Impl.erase! k t).maxKey?.isSome = true → t.maxKey?.isSome = true