∀ {α : Type u} {cmp : α → α → Ordering} {β : Type v} {t : Std.DTreeMap α (fun x => β) cmp} {f : α → β → Bool},
  Std.DTreeMap.Const.toList (Std.DTreeMap.filter f t) = List.filter (fun p => f p.1 p.2) (Std.DTreeMap.Const.toList t)