Benedict of Pope Emeritus. I believe he can fill in.
of = is
It doesn’t matter what you believe. If bergoglio resigns, the College of Cardinals will have an enclave, and elect a new pope. They could elect Benedict again, but 1. the office wouldn’t somehow revert back to him; 2. I’m not sure he’d take it; and 3. at his advantaged it is doubtful he’d have the strength or energy to clean up the mess.