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