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