∀ {α : Type u} {β : Type v} {cmp : α → α → Ordering} {t : Std.ExtTreeMap α β cmp} [inst : Std.TransCmp cmp] {k : α}
  {f : Option β → Option β}, k ∉ t → (f t[k]?).isNone = true → (t.alter k f).size = t.size