∀ {α : Type u} {cmp : α → α → Ordering} {β : Type v} {t : Std.DTreeMap α (fun x => β) cmp} [Std.TransCmp cmp]
  [inst : Std.LawfulEqCmp cmp] {a : α} {fallback : β}, Std.DTreeMap.Const.getD t a fallback = t.getD a fallback