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