Due to an issue, I temporarily mapped another key to where the PROG key was. Now I can’t figure out how to remap it as it seems to appear nowhere in the list? 
You may find it by looking at the default Qwerty keymap (and becarefully NOT saving it )
Happy hacking !
Thanks for the hint, looks like it is only mapped as “Transparent” there either…
I think It’s ok to map any key to the prog key. Otherwise ___ (transparent) is fine too.
I need my prog key to do double duty on my Atreus because there are so few keys: it locks the function layer and it has the butterfly symbol printed on it. I also use it on my Model01 where it’s a leader key.
IOW it still works as the prog key whatever is in your keymap. It can have two functions.