∀ {α : Type u} {β : α → Type v} {cmp : α → α → Ordering} {t : Std.DTreeMap.Raw α β cmp} [Std.TransCmp cmp]
  {f : α → Bool} {k : α}, t.WF → (Std.DTreeMap.Raw.filter (fun k x => f k) t).getKey? k = Option.filter f (t.getKey? k)