Function both_ways 

fn both_ways(
    client: &Client,
    dir: &Path,
    listed: &Listed,
    state: &mut State,
    state_path: &Path,
    report: &mut Report,
    first: bool,
    until: Option<Instant>,
) -> Result<bool, SyncError>

What changed here, sent (on a collection’s first turn); then, unless nothing changed on either side, what changed there, brought until until. The ctag is kept once both went through whole; what the server refused of an item is said once the rest is done. Whether it is whole.