How to create multiple 'aggregate' facts on a 'parent' object calculated from child objects (composition)

I have object(s) that will introduce facts that also have 'child' objects (composition). There will be multiple facts that are aggregate values on the parent that are calculated from the 'children.'

Typically these aggregate fact on the parent will follow one of the following patterns:

- count of children matching a filter (or all)
- sum of children matching a filter (or all)

I didn't see anything like that in the examples. It would be great if it could be added!

Start by reviewing the repository's existing examples and the requirements listed here: parent facts calculated from composed children, including filtered or unfiltered counts and sums. Determine whether the rules engine already supports this pattern; done requires a concrete example or a narrowed implementation proposal that defines the supported aggregate behavior.

Written by the indexing model from the issue text.