trying to write the network code for that "what are the top relays used by people who follow or are followed by user X?" tool.

the simplest problem to solve is "who is following user X on which relays?" because you just ask every relay for the contact lists that tag (follow) that user, and each contact list will contain a relay list, so you just aggregate that.

it gets a little bit more complicated the other way around, because first you need to perform a query to find the user's contact list, and then you need to query all the relays once again for the contact lists of that are tagged in that contact list, and then aggregate that.