∀ {α : Type u} {β : Type v} [inst : Ord α] (t : Std.DTreeMap.Internal.Impl α fun x => β) (k : α) (v : β)
  (ht : t.Balanced),
  Std.DTreeMap.Internal.Impl.Const.getThenInsertIfNew? t k v ht =
    match Std.DTreeMap.Internal.Impl.Const.get? t k with
    | none => (none, (Std.DTreeMap.Internal.Impl.insertIfNew k v t ht).impl)
    | some b => (some b, t)