Complex sentence: Rules for deciding unreachability are chosen so that rule-based unreachability is a sufficient condition for true unreachability, that is, if the rules say a statement is unreachable, then it can be proven mathematically that the statement is truly unreachable.
Split into simpler sentences: Rules for deciding unreachability are chosen so that rule-based unreachability is a sufficient condition for true unreachability. That is, if the rules say a statement is unreachable, then it can be proven mathematically that the statement is truly unreachable.