Additional exercise for Chapter 6 -- Finding cliques in a graph.
A clique is a subset of a graph such that each of its vertices
is connected by an edge to all the other vertices in the clique.
Note that each node of a graph is a clique of size 1 and that the empty
set is also a clique.
Write an ASP program which finds all cliques in a given undirected graph;
i.e., each answer set will correspond to one clique in the graph.
Bonus: Look up the #count aggregate in the manual for your
solver of choice and use it to modify your program to only find
cliques of size >= 3.