Problem description
-------------------
Let G=<V,E> a directed graph, where each edge is given
a color. Does there exist a non-empty subset S of V that:

 1. given any pair of vertex v1 and v2 n S, there is no 
    monochrome path between v1 and v2.
 2. (maximality) for any vertex v not in S, there exists 
    a monochrome path connecting v to some other node in S.


Input format
------------
A set of facts of the form
    vertex(V).  		<-- vertices
    edge(V1,V2,Color). 		<-- colored edges


Output format
-------------
a set of facts of the form
    in(X).			<-- selected nodes
if no instances of "in" are given, the answer is supposed
to be "NO".