/** User drills down on 'Publish Year/2010'. */
private FacetResult drillDown() throws IOException {
DirectoryReader indexReader = DirectoryReader.open(indexDir);
IndexSearcher searcher = new IndexSearcher(indexReader);
SortedSetDocValuesReaderState state = new DefaultSortedSetDocValuesReaderState(indexReader);
// Now user drills down on Publish Year/2010:
DrillDownQuery q = new DrillDownQuery(config);
q.add("Publish Year", "2010");
FacetsCollector fc = new FacetsCollector();