• Symmetric Strict Monoidality With Respect to Unions. The inverse image function of Item 1 has a symmetric strict monoidal structure
    \[ \webleft (f^{-1},f^{-1,\otimes },f^{-1,\otimes }_{\mathbb {1}}\webright ) \colon \webleft (\mathcal{P}\webleft (B\webright ),\cup ,\emptyset \webright ) \to \webleft (\mathcal{P}\webleft (A\webright ),\cup ,\emptyset \webright ), \]

    being equipped with equalities

    \[ \begin{gathered} f^{-1,\otimes }_{U,V} \colon f^{-1}\webleft (U\webright )\cup f^{-1}\webleft (V\webright ) \mathbin {\overset {=}{\rightarrow }}f^{-1}\webleft (U\cup V\webright ),\\ f^{-1,\otimes }_{\mathbb {1}} \colon \emptyset \mathbin {\overset {=}{\rightarrow }}f^{-1}\webleft (\emptyset \webright ), \end{gathered} \]

    natural in $U,V\in \mathcal{P}\webleft (B\webright )$.


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