public void assignActive(final TaskId task) {
        assertNotAssigned(task);
        assignedActiveTasks.taskIds().add(task);
    }
@Test
    public void shouldRefuseDoubleActiveTask() {
        final ClientState clientState = new ClientState(1);
        clientState.assignActive(TASK_0_0);
        assertThrows(IllegalArgumentException.class, () -> clientState.assignActive(TASK_0_0));
    }