private Set<String> extractVars(String query) {
   log.debug(" {}", query);
   Query q = QueryFactory.create(query);
   Element element = q.getQueryPattern();
   ElementWalker.walk(element, collector);
   Set<String> vars = collector.getVariables();
   log.debug(" > {}", vars);
   return vars;
 }
 @After
 public void after() {
   collector.reset();
 }