∀ {α : Type u} {cmp : α → α → Ordering} {β : Type v} {t : Std.DTreeMap α (fun x => β) cmp} [Std.TransCmp cmp] {k : α},
  (Array.find? (fun x => cmp x.1 k == Ordering.eq) (Std.DTreeMap.Const.toArray t) = none) = (k ∉ t)