不允許新增強制屬性到現存類別、從現存類別刪除強制屬性、或將選擇性屬性新增到不是 backlink 屬性 (直接或透過繼承，例如新增或刪除輔助類別) 的特殊類別端。

Adding a new mandatory attribute to an existing class, deleting a mandatory attribute from an existing class, or adding an optional attribute to the special class Top that is not a backlink attribute (directly or through inheritance, for example, by adding or deleting an auxiliary class) is not allowed.