diff --git a/examples/graph-coloring.lp b/examples/graph-coloring.lp
index de5d382..53fef3b 100644
--- a/examples/graph-coloring.lp
+++ b/examples/graph-coloring.lp
@@ -3,7 +3,8 @@
 #external vertex(1).
 #show color/2.
 
-{color(V,C)} :- vertex(V), color(C).
+{color(V, C)} :- vertex(V), color(C).
 covered(V) :- color(V, _).
 :- vertex(V), not covered(V).
-:- color(V1,C), color(V2,C), edge(V1,V2).
+:- color(V1, C), color(V2, C), edge(V1, V2).
+:- color(V, C1), color(V, C2), C1 != C2.