Complete Name: 3-Colorability Problem.

Input Predicates: node/1, arc/2

Output Predicates: col/2

Problem Description: Graph 3-colorability is a hard (NP-complete) problem. 
Given a graph G (node names are encoded in the input predicate node/1, 
while arcs are encoded in the binary predicate arc/2), 
a solution to the problem is to encode in col/2 an assignment of 
one of the three colors 'red', 'green' and 'blue', to nodes of G such that 
no couple of nodes X,Y, for which arc(X,Y) holds, has the same color, i.e.
col(X,C) and col(Y,C) both hold, where C is one among {red,green,blue}.


There are 2 corner-case scenarios: 
- there is no solver that is able to find a solution for the provided instances in 10 minutes (competition timeout)  ===> Maybe they are too hard
- the provided instances are very easy  ===> Maybe they are not worth of attention

Please provide in this file some argument  showing that both (1) and (2) do not apply to your case.

e.g. mention how the systems behaved in the last competition on your instances (if you are resubmitting something).
or say "I could solve some of them in my machine (2.4GHz core 2) by exploiting DLV with average running time of X minutes)".
