How are CBOE SPX PM-settled options settled if the S&P 500 is limit down at the time of expiration, i.e. a 20% Level 3 circuit breaker is tripped and all trading has halted? Do they settle as usual using the frozen limit price as the index price, or are there alternate procedures in place? Did some serious googling and couldn't find anything on this admittedly arcane but possible scenario.