∀ {α : Type u} {cmp : α → α → Ordering} {β : Type v} {t : Std.DTreeMap.Raw α (fun x => β) cmp} [Std.TransCmp cmp]
  {f : α → β → Bool},
  t.WF →
    ((Std.DTreeMap.Raw.filter f t).isEmpty = false ↔
      ∃ k, ∃ (h : k ∈ t), f (t.getKey k h) (Std.DTreeMap.Raw.Const.get t k h) = true)