Github pull request:
 
In case Proprietary Relicensing happens, copyleft is effectively wiped out.
What remains is a fallback license of sorts, which should at least require attribution I think...
I wonder if this is enough, I can't figure out well enough what would happen when all the 
fun questions would be asked (compatibility...)
 
Reference:
https://github.com/richardfontana/copyleft-next/pull/17