∀ {α : Type u} {cmp : α → α → Ordering} [Std.TransCmp cmp] {β : Type v} {t : Std.DTreeMap α (fun x => β) cmp} {k : α}
  {f : β → β}, (Std.DTreeMap.Const.modify t k f).getKey? k = if k ∈ t then some k else none