@Override
        public void onVisitorStatistics(VisitorStatistics vs) {
            super.onVisitorStatistics(vs);
            builder.append("onVisitorStatistics : ");
            // Only bother with a couple of fields.
            builder.append(vs.getBucketsVisited()).append(" buckets visited, ");
            builder.append(vs.getDocumentsReturned()).append(" docs returned\n");
        }
Test that using an id.user=foo selection only tries to visit a single
superbucket for that user.
documentapi/src/test/java/com/yahoo/documentapi/messagebus/MessageBusVisitorSessionTestCase.java