void unassignStandby(final TaskId task) {
        final Set<TaskId> taskIds = assignedStandbyTasks.taskIds();
        if (!taskIds.contains(task)) {
            throw new IllegalArgumentException("Tried to unassign standby task " + task + ", but it is not currently assigned: " + this);
        }
        taskIds.remove(task);
    }
@Test
    public void shouldRefuseToUnassignNotAssignedStandbyTask() {
        final ClientState clientState = new ClientState(1);
        assertThrows(IllegalArgumentException.class, () -> clientState.unassignStandby(TASK_0_0));
    }