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