∀ {α : Type u} {cmp : α → α → Ordering} {β : Type v} {t : Std.DTreeMap α (fun x => β) cmp} [Std.TransCmp cmp] {k : α}
  {v : β}, Std.DTreeMap.Const.get? t k = some v ↔ ∃ k', cmp k k' = Ordering.eq ∧ (k', v) ∈ Std.DTreeMap.Const.toList t