∀ {α : Type u} {instOrd : Ord α} {β : Type v} {t : Std.DTreeMap.Internal.Impl α fun x => β} [Std.TransOrd α],
  t.WF →
    ∀ {a : α} {fallback : β},
      Std.DTreeMap.Internal.Impl.contains a t = false → Std.DTreeMap.Internal.Impl.Const.getD t a fallback = fallback