Function past_below 

fn past_below(
    folder: &Path,
    computer: &str,
    round: u32,
    full_round: u32,
    readers: &BTreeSet<String>,
) -> u32

The round below which a computer’s rounds are past their time: every reader (readers) read past them (its notes say a later round: it met their end, Memory::met, or read on from a later full round; a reader without notes in the folder read none), and never its last full round nor the rounds continuing it: a device joining reads from the oldest kept, and finds every entry there (share::rounds_tests). A device forgotten that comes back counts again at once, its notes with it, and reads from the oldest kept the same way.