Hi Larry,
Thank you for the email. Unfortunately the fast exchange equation has not been implemented in GUARDD (at least not to my knowledge).
For what it's worth, I indicated some basic steps of how this could be implemented
Based on how I programmed GUARDD, coding the fast exchange equation fitting process would not be difficult. However, there would be a lot of "loose ends" to clean up in order to display results properly in all the program's outputs (e.g., figures, chi2 space, output table).
Also, due to my own time limitations, I don't expect that I myself can code this module any time soon...but if you're interested to give it a try, then I'm happy to provide some support / advice.
Sorry it may not be the answer you're looking for, but I hope that helps.
Regards,
-Ian