Recall from **Definition 3.26** that every category \$$\mathcal{C}\$$ has an opposite \$$\mathcal{C}^{op}\$$.
Let \$$F : \mathcal{C} \rightarrow \mathcal{D} \$$ be a functor.
How should we define its opposite, \$$F^{op} : \mathcal{C}^{op} \rightarrow \mathcal{D}^{op} \$$?
That is, how should \$$F^{op} \$$ act on objects, and how should it act on morphisms?

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