∀ {α : Type u} {cmp : α → α → Ordering} {β : Type v} {t : Std.ExtDTreeMap α (fun x => β) cmp} [inst : Std.TransCmp cmp]
  {k a : α} {fallback v : β},
  Std.ExtDTreeMap.Const.getD (t.insert k v) a fallback =
    if cmp k a = Ordering.eq then v else Std.ExtDTreeMap.Const.getD t a fallback