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