∀ {α : Type u} {instOrd : Ord α} {β : Type v} {γ : Type w} {t : Std.DTreeMap.Internal.Impl α fun x => β}
  [Std.TransOrd α] {f : α → β → Option γ} {k k' : α} {fallback : γ},
  t.WF →
    t.getKey? k = some k' →
      Std.DTreeMap.Internal.Impl.Const.getD (Std.DTreeMap.Internal.Impl.filterMap! f t) k fallback =
        ((Std.DTreeMap.Internal.Impl.Const.get? t k).bind fun x => f k' x).getD fallback