ex_max_clique.txt -- Additional exercise for Chapter 6, finding a maximum 
 clique in a graph.
 Last Modified: 4/11/14
 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.
 A maximum clique is a clique of the largest possible size in a given graph.
 Write a CR-Prolog program which finds a maximum clique in a given undirected 
 graph.