∀ {α : Type u} {β : α → Type v} {instOrd : Ord α} {t : Std.DTreeMap.Internal.Impl α β} [Std.TransOrd α]
  [inst : Std.LawfulEqOrd α],
  t.WF → ∀ {k : α} {f : β k → β k}, (Std.DTreeMap.Internal.Impl.modify k f t).getKey? k = if k ∈ t then some k else none