Idea: check for overlap down to the leaf rules

[hawkrives]

Currently, we only check overlap between the top-level requirements (because my naive implementation would only check against sibling rules).

In order to defeat this, we need to (at the top level) somehow recurse into each rule, and provide the possible items into the nested levels as we check them.

Q: Does that actually get us anything? I think it does, because you should be able to reduce the number of iterations in nested Count rules…