∀ {α : Type u} {cmp : α → α → Ordering} {β : Type v} {t : Std.ExtDTreeMap α (fun x => β) cmp} [inst : Std.TransCmp cmp]
  [inst_1 : Inhabited β] {a : α}, t.contains a = false → Std.ExtDTreeMap.Const.get! t a = default