∀ {α : Type u} {β : α → Type v} {cmp : α → α → Ordering} {t : Std.DTreeMap.Raw α β cmp} [Std.TransCmp cmp],
  t.WF →
    ∀ {k : α} {v : β k} {fallback : α},
      (t.insertIfNew k v).maxKeyD fallback = t.maxKey?.elim k fun k' => if cmp k' k = Ordering.lt then k else k'