∀ {α : Type u} {cmp : α → α → Ordering} {β : Type v} {γ : Type w} {t : Std.ExtDTreeMap α (fun x => β) cmp}
  [inst : Std.TransCmp cmp] [Std.LawfulEqCmp cmp] {f : α → β → γ} {k : α},
  Std.ExtDTreeMap.Const.get? (Std.ExtDTreeMap.map f t) k = Option.map (f k) (Std.ExtDTreeMap.Const.get? t k)