[rules-users] correlating two collects

I am finally having some time to use collects. What I basically need to do is to collect items from a list using a specific criteria, then collect items from a diff list using another criteria, and finally compare the groups collected using a 3rd criteria Here is what I am thinking: i : Item(type==A) ig : ItemGroup(size>1) collect Item(a==i.a, b==i.b, ... n==i.n) i2: Item(type==B) ig2 : ItemGroup( size>1, ga==ig.ga, gb==i.gb, .... ) collect Item(a==i.a, b==i.b, ... n==i.n) 1) It is quite nice that any implementation of java.util.Collection can be the resut of collect. ItemGroup here not only keeps hold of all Items returned but has some aggregate properties (afaik, I cannot use the aggregate functions with accumulate since I have multiple column aggregation). 2) Am I on the right track? Or is there a better way? 3) Is the collect recomputed from scratch or incrementally every time a new fact is asserted or updated? thanks