∀ {α : Type u} {cmp : α → α → Ordering} {β : Type v} {t : Std.DTreeMap.Raw α (fun x => β) cmp} [Std.TransCmp cmp],
  t.WF →
    ∀ {k k' : α} {fallback : β} {f : Option β → Option β},
      Std.DTreeMap.Raw.Const.getD (Std.DTreeMap.Raw.Const.alter t k f) k' fallback =
        if cmp k k' = Ordering.eq then (f (Std.DTreeMap.Raw.Const.get? t k)).getD fallback
        else Std.DTreeMap.Raw.Const.getD t k' fallback