∀ {α : Type u} {β : α → Type v} {cmp : α → α → Ordering} {t : Std.DTreeMap.Raw α β cmp} [Std.TransCmp cmp],
  t.WF →
    ∀ {k a fallback : α} {v : β k},
      (t.insert k v).getKeyD a fallback = if cmp k a = Ordering.eq then k else t.getKeyD a fallback