∀ {α : Type u} {cmp : α → α → Ordering} {β : Type v} {γ : Type w} {t : Std.DTreeMap α (fun x => β) cmp}
  [inst : Std.TransCmp cmp] {f : α → β → γ} {k : α} {fallback : γ},
  Std.DTreeMap.Const.getD (Std.DTreeMap.map f t) k fallback =
    (Option.pmap (fun v h => f (t.getKey k h) v) (Std.DTreeMap.Const.get? t k) ⋯).getD fallback