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