public Iterator join(Iterator[] selections) {
		HashSet result = new HashSet();

		for (int i = 0; i < selections.length; i++) {
			PersistentIterator iterator = (PersistentIterator) selections[i];
			int oid;

			while ((oid = iterator.nextOid()) != 0) {
				result.add(new Integer(oid));
			}
		}

		return new HashIterator(result);
	}