Basically a rules engine let's me make declarations like "I am seeing this message, what do I want to do with it for handling" without writing eight million individual handlers.

If this were a serious project I would go out and grab someone else's and call it a day (or maybe #rego), but because this is a toy and I'm experimenting I'm going to create a little DSL—based on #datalog—and see if I can use that to implement some ideas in this space. I'll call it #APRL or #ActivityPubRulesLanguage.