Function explain_reply 

pub fn explain_reply(
    r: Reply,
    commands: impl FnOnce() -> Vec<CommandInfo>,
) -> Reply

explain for a reply that came from elsewhere (a running app, a worker): an error whose detail.kind is unknown or bad_params (or whose message starts with “unknown command”) and that is not explained yet. commands is only called when there is something to explain. Anything else comes back unchanged.