Whether the case leaves the simulation before an actual screening at the
current screening age is determined by the member tt(Loop::leaving).  In the
original program this is determined as follows:
    itemization(
    it() the case's natural death occurs before the screening round's age,
    it() or there is a tumor and the tumor's self-detection age is before the
        screening round's age
    )

Converting this condition, then the case leaves the simulation if
    itemization(
    it() the screening round's age is at most equal to the case's natural
        death age, and
    it() there is no tumor or the screening round's age is at most equal to
        the tumor's self-detection age.
    )

If at this point the case hasn't left the simulation, then
    itemization(
    it() if there is a tumor and the tumor's self-detection age is at most
        equal to the case's natural death age then an interval cancer has
        occurred (handled by tt(Loop::intervalCancer));
    it() otherwise, the case's state is set to tt(LEFT_DURING) as the
        case's natural death age has caused the case to leave the simulation.
        However, a tumor might still have developed at that point, and if so
        the tumor's characteristics are determined at the case's natural death
        age (cf. tt(Tumor::characteristics), section ref(TUMOR)).
    )

