Function keep_others_for_their_addresses 

pub fn keep_others_for_their_addresses() -> Result<usize, String>

Once on each device (state/pgp-others.toml): each key of others/ kept without what it serves (format 1) gets it, as it is judged now (judged): its one address when every user ID it carries names it and it is bound, else none; written as judged, not learned. Kept so, an older Sioul’s later write to it (every user ID of a header, merged) adds no address it serves. The key’s own file is never touched: one that serves none stays, to serve again once it comes for an address (its owner’s next message, a lookup). How many keys it wrote it for.