[001U] Exercise 3.2·a.

Suppose that $B$ is an internal category in $\mathbf{Set}$, i.e. it has a set of objects. Exhibit an equivalence of displayed categories between $P\Sub{\bullet}$ as described above, and the naïve definition which $E\Sub{x}$ is the collection of objects $u\in E$ such that $Pu = x$.