∀ {α : Type u} {cmp : α → α → Ordering} [inst : Std.TransCmp cmp] {β : Type v} {t : Std.ExtDTreeMap α (fun x => β) cmp}
  {k k' : α} {f : β → β},
  Std.ExtDTreeMap.Const.get? (Std.ExtDTreeMap.Const.modify t k f) k' =
    if cmp k k' = Ordering.eq then Option.map f (Std.ExtDTreeMap.Const.get? t k) else Std.ExtDTreeMap.Const.get? t k'