∀ {α : Type u} {cmp : α → α → Ordering} {β : Type v} {t : Std.ExtDTreeMap α (fun x => β) cmp} [inst : Std.TransCmp cmp]
  [Std.LawfulEqCmp cmp] {k : α} {v : β},
  (k, v) ∈ Std.ExtDTreeMap.Const.toList t ↔ Std.ExtDTreeMap.Const.get? t k = some v