public void union(PartitionIdSet other) {
        this.bitSet.or(other.bitSet);
        resetSize();
    }
@Test
    public void test_union_whenSetsOverlap() {
        partitionIdSet = new PartitionIdSet(11, listOf(0, 1, 2, 3, 4));
        PartitionIdSet other = new PartitionIdSet(11, listOf(0, 1, 2));
        partitionIdSet.union(other);
        assertContents(partitionIdSet);
    }