∀ {α : Type u} {β : α → Type v} {cmp : α → α → Ordering} {t : Std.DTreeMap.Raw α β cmp} [Std.TransCmp cmp]
  [inst : Std.LawfulEqCmp cmp] {f : (a : α) → β a → Bool} {k : α},
  t.WF → (Std.DTreeMap.Raw.filter f t).contains k = Option.any (f k) (t.get? k)