∀ {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} {f : β → β} {fallback : α},
  (Std.ExtDTreeMap.Const.modify t k f).maxKeyD fallback = t.maxKeyD fallback