∀ {α : Type u} {β : Type v} {cmp : α → α → Ordering} {t : Std.TreeMap.Raw α β cmp} [Std.TransCmp cmp],
  t.WF → ∀ {k k' fallback : α}, cmp k k' = Ordering.eq → t.getKeyD k fallback = t.getKeyD k' fallback