• Interaction With Symmetric Differences. The diagram

    does not commute in general, i.e. we may have

    \[ \bigcap _{W\in \mathcal{U}\mathbin {\triangle }\mathcal{V}}W\neq \webleft(\bigcap _{U\in \mathcal{U}}U\webright)\mathbin {\triangle }\webleft(\bigcap _{V\in \mathcal{V}}V\webright) \]

    in general, where $\mathcal{U},\mathcal{V}\in \mathcal{P}\webleft (\mathcal{P}\webleft (X\webright )\webright )$.


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