[rules-users] Using "collect" repeatedly over sub-groups of a fact

I have a single rule that includes a "collect" pattern to pass a collection to the RHS. However, what I now need to do fire this rule for multiple collections based on groupings defined by attributes on the fact. Here is an example: Fact(a = 1, b = 1, c = 1) Fact(a = 1, b = 1, c = 2) Fact(a = 2, b = 1, c = 3) Fact(a = 2, b = 2, c = 4) Fact(a = 2, b = 2, c = 5) I would like to create sub-groups based on the attributes 'a' and 'b'. My rule should fire three times for the facts listed above, providing the corresponding collection of Fact. The only solution that I can think of is to build a HashSet and add each Fact to the HashSet using the attributes as keys. I suppose it would have to be a nested set since I can't define a multi-valued key class within a rule. I could then handle this HashSet in a RHS block. However, I realize that I'm completely subverting the whole rule languageŠ Any suggestions? Thanks, Mike