∀ {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 → t.isEmpty = false → ∀ {fallback : α}, t.getKey! (t.maxKeyD fallback) = t.maxKeyD fallback