∀ {α : Type u} {β : Type v} {cmp : α → α → Ordering} {t : Std.TreeMap.Raw α β cmp} [Std.TransCmp cmp],
  t.WF → ∀ {k : α} {f : Option β → Option β}, k ∉ t → (f t[k]?).isSome = true → (t.alter k f).size = t.size + 1