Dynamic user lists

[ShreyanJain9]

User lists drive a lot of systems (threadgates, feeds). It would be great if they could be derived from existing data using queries, eg a "mutuals" list (people who follow me and that I follow) or a "friends of friends" list (people followed by people I follow). This would help drive larger communities in the network

Here's how I think it could be done:

• A common format for "user lists" (not the same as app.bsky.graph.list) that could be used to represent a list of users when returned by an endpoint. So you could represent app.bsky.graph.list with it, or a user's follower list, or a user's following list
• A lexicon of some sort, perhaps similar to Skyfeed Builder's JSON blocks, that allow representing "virtual lists" in terms of other existing lists. i.e. you could define a user's "Mutuals" list as the intersection of their Following and Follower lists. (Virtual lists would need to have the ability to be aware of their viewer. Or they could be defined like a function, where they can take any number of users and one of them is implicitly the viewer)

Thoughts?