Function card_known_until 

fn card_known_until(
    knowledge: &Knowledge,
    due: i64,
    from: i64,
    until: i64,
) -> i64

Until when a dose due at due stays known not marked elsewhere, seen from from (doubts_now: each other device fresh five minutes after it was last heard in full): until when nothing ends it before; 0 when it is not known at from.