∀ {α : Type u} {β : Type v} {cmp : α → α → Ordering} {t : Std.TreeMap.Raw α β cmp} [Std.TransCmp cmp]
  [inst : Inhabited α],
  t.WF → ∀ {k : α} {v : β}, (t.insert k v).maxKey! = t.maxKey?.elim k fun k' => if (cmp k' k).isLE = true then k else k'