List<Integer> originalReplicas() {
        List<Integer> replicas = new ArrayList<>(this.replicas);
        replicas.removeAll(adding);
        return replicas;
    }
@Test
    public void testOriginalReplicas() {
        PartitionReassignmentReplicas replicas = new PartitionReassignmentReplicas(
            partitionAssignment(Arrays.asList(0, 1, 2)), partitionAssignment(Arrays.asList(0, 1, 3)));
        assertEquals(Arrays.asList(0, 1, 2), replicas.originalReplicas());
    }