∀ {α : Type u} {cmp : α → α → Ordering} {β : Type v} {t : Std.DTreeMap α (fun x => β) cmp} [Std.TransCmp cmp]
  [inst : Inhabited α] {k k' : α} {f : Option β → Option β},
  (Std.DTreeMap.Const.alter t k f).getKey! k' =
    if cmp k k' = Ordering.eq then if (f (Std.DTreeMap.Const.get? t k)).isSome = true then k else default
    else t.getKey! k'