public Set<TaskId> standbyTasks() {
        return unmodifiableSet(assignedStandbyTasks.taskIds());
    }
@Test
    public void shouldNotModifyStandbyView() {
        final ClientState clientState = new ClientState(1);
        final Set<TaskId> taskIds = clientState.standbyTasks();
        assertThrows(UnsupportedOperationException.class, () -> taskIds.add(TASK_0_0));
        assertThat(clientState, hasStandbyTasks(0));
    }