DSet set1a = (DSet) database.lookup(set_1);
DSet set2a = (DSet) database.lookup(set_2);
// check looked up sets
assertTrue(set1a.containsAll(set1));
assertTrue(set2a.containsAll(set2));
// now TestThreadsNLocks set operations:
DSet set3 = set1.difference(set2);
assertEquals(1, set3.size());