• Functoriality. The assignment $U\mapsto R_{!}\webleft (U\webright )$ defines a functor
    \[ R_{!}\colon \webleft (\mathcal{P}\webleft (A\webright ),\subset \webright )\to \webleft (\mathcal{P}\webleft (B\webright ),\subset \webright ) \]

    where

    • Action on Objects. For each $U\in \mathcal{P}\webleft (A\webright )$, we have

      \[ \webleft [R_{!}\webright ]\webleft (U\webright )\mathrel {\smash {\overset {\mathclap {\scriptscriptstyle \text{def}}}=}}R_{!}\webleft (U\webright ); \]

    • Action on Morphisms. For each $U,V\in \mathcal{P}\webleft (A\webright )$:
      • If $U\subset V$, then $R_{!}\webleft (U\webright )\subset R_{!}\webleft (V\webright )$.


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