∀ {α : Type u} {cmp : α → α → Ordering} {β : Type v} {γ : Type w} {t : Std.ExtDTreeMap α (fun x => β) cmp}
  [inst : Std.TransCmp cmp] [Std.LawfulEqCmp cmp] {f : α → β → Option γ} {k : α} {fallback : γ},
  Std.ExtDTreeMap.Const.getD (Std.ExtDTreeMap.filterMap f t) k fallback =
    ((Std.ExtDTreeMap.Const.get? t k).bind (f k)).getD fallback