∀ {α : Type u} {cmp : α → α → Ordering} {β : Type v} {t : Std.DTreeMap α (fun x => β) cmp} [Std.TransCmp cmp] {k : α}
  {f : β → β} {fallback : α},
  cmp ((Std.DTreeMap.Const.modify t k f).maxKeyD fallback) (t.maxKeyD fallback) = Ordering.eq