∀ {α : Type u} {instOrd : Ord α} {β : Type v} {t : Std.DTreeMap.Internal.Impl α fun x => β} [Std.TransOrd α]
  {f : α → β → Bool} {k k' : α} (h : t.WF),
  t.getKey? k = some k' →
    Std.DTreeMap.Internal.Impl.Const.get? (Std.DTreeMap.Internal.Impl.filter f t ⋯).impl k =
      Option.filter (fun x => f k' x) (Std.DTreeMap.Internal.Impl.Const.get? t k)