@Override
 public <T extends Identifiable> Stream<T> list(Class<T> clazz, String filter) {
   final Predicate<Object> accept =
       (Predicate<Object>) Tql.parse(filter).accept(new ObjectPredicateVisitor(clazz));
   return source(clazz).filter(accept);
 }
 @Override
 public <T extends Identifiable> boolean exist(Class<T> clazz, String filter) {
   final Predicate<Object> accept =
       (Predicate<Object>) Tql.parse(filter).accept(new ObjectPredicateVisitor(clazz));
   return source(clazz).filter(accept).findAny().isPresent();
 }