Question 3

The intuition is that negation for finite goals can be implemented in this case by computing the set of all the solutions and then checking membership. For a ground goal $\neg p(\bar{t})$, having a finite derivation tree, we have:

$$\neg p(\bar{t}) \leftrightarrow \bar{t} \in \{\bar{X} \vert p(\bar{X})\}$$

where the intensional set construction on the right can be directly expressed using the complete implementation of setof available in the assumption of the problem.