Update examples

This updates the examples to showcase the scope of anthem’s feature set.
New examples are added concerning placeholders, hiding predicates, and
simplifications related to integer variables.

examples/graph-coloring.lp

@@ -1,10 +1,18 @@
-#external color(1).
-#external edge(2).
-#external vertex(1).
-#show color/2.
-
+% assign a set of colors to each vertex
 {color(V, C)} :- vertex(V), color(C).
+
+% at most one color per vertex
+:- color(V, C1), color(V, C2), C1 != C2.
+
+% at least one color per vertex
 covered(V) :- color(V, _).
 :- vertex(V), not covered(V).
+
+% adjacent vertices don’t share the same color
 :- color(V1, C), color(V2, C), edge(V1, V2).
-:- color(V, C1), color(V, C2), C1 != C2.
+
+#show color/2.
+
+#external vertex(1).
+#external edge(2).
+#external color(1).