[rules-users] Inter-fact comparison

Greetings all, I have a requirement to compare a set of facts against each other, calculate a value that indicates the amount of difference (Levenshtein distance) between a single attribute on each fact. So I load all of my facts into working memory then what? Do I execute a rule first that creates a new set of facts which is a Cartesian product of the original set of facts, each new fact containing a reference to two original facts and the diff between them? Looked at using a custom accumulate function but I don't think that's going to do what I need since it is intended to go through a set of facts and return a single value. Any and all related help is appreciated. Earnie!