Talk:Set Theory/Relations

From the page: $$ A \times B = \{(a,b) \in \mathcal{P}\mathcal{P}(A\cup B) \mid a \in A \wedge b \in B\}$$

The $$\mathcal{P}\mathcal{P}$$ notation is not defined in previous sections of this Wikibook. Is that the power set of a power set, or a typo? Not clear.