[rules-users] rule algorithm

Hello, I search a rule implementation for the following calculation: I have n Objects of differents types (A, B, C, ...) each with a parameter certainty. the overall certainty is calculated as follows: certaintyOverall = (certaintyA * weightA + certaintyB * weightB + ... ) / (weightA + weightB + ...) If it can be assumed that all Objects in the formula always exist it's quite easy: when A (certaintyA: certainty) B (certaintyB: certainty) ... then certaintyOverall = (certaintyA * weightA + certaintyB * weightB + ... ) / (weightA + weightB + ...) But if one ore more objects are missing the formula should be reduced to the certainty and weights of the existing objects. But there are too many combinations of existing/nonexisting objects. How can this be implemented in Drools? Regards Thomas