title: Comparing AICc of ARIMA models produced in fable
body: <p>When using <code>fable</code> to produce a set of different <code>ARIMA</code> models using different combinations of xregs, if different models choose a different <code>d</code> and <code>D</code> parameters then the <code>AICc</code> are no longer comparable, right?  In that case, should I just find the max <code>d</code> and <code>D</code> chosen from all the models and fix those parameters and retrain the models for comparison?</p>