一句话：证明可数幂集函子终链在 ω₁ 处含无后继的幽灵元素，故该迁移系统非强外延，并推广至其他子函子。
论文研究可数幂集函子的终链中索引 ω₁ 处的集合结构。将该集合视为迁移系统时，作者构造了一个“幽灵”元素：它没有后继，但在每个后继索引处的分量都非空，因此该迁移系统不是强外延的。方法改编自 Forti 与 Honsell 的构造，并推广到幂集函子的其他子函子，给出更大序数处的幽灵元素。这些结果进一步精确刻画了此类终链中哪些集合是强外延的。
In the final chain of the countable powerset functor, we show that the set at index $\omega_1$, regarded as a transition system, is not strongly extensional because it contains a "ghost" element that has no successor even though its component at each successor index is inhabited. The method, adapted from a construction of Forti and Honsell, also gives ghosts at larger ordinals in the final chain of other subfunctors of the powerset functor. This leads to a precise description of which sets in these final chains are strongly extensional.