∀ {α : Type u} {instOrd : Ord α} {β : Type v} {t : Std.DTreeMap.Internal.Impl α fun x => β} [Std.TransOrd α],
  t.WF →
    ∀ {k : α} {f : Option β → Option β},
      k ∈ t →
        (f (Std.DTreeMap.Internal.Impl.Const.get? t k)).isNone = true →
          (Std.DTreeMap.Internal.Impl.Const.alter! k f t).size = t.size - 1