∀ {α : Type u} {cmp : α → α → Ordering} [Std.TransCmp cmp] {β : Type v} {t : Std.DTreeMap.Raw α (fun x => β) cmp},
  t.WF →
    ∀ {k k' fallback : α} {f : β → β},
      (Std.DTreeMap.Raw.Const.modify t k f).getKeyD k' fallback =
        if cmp k k' = Ordering.eq then if k ∈ t then k else fallback else t.getKeyD k' fallback