Function keep 

fn keep(
    state: &mut AccountState,
    state_path: &Path,
    dir: &Path,
    folder: &str,
    records: &[Record],
    unreadable: &[u32],
    update: &mut Update,
) -> Result<(), LearnError>

One batch kept: its records appended to the folder’s file, their UIDs marked done; those not kept (the server could not give them, or All Mail leaves them out) marked skipped; where the folder stopped saved.