Following the pre-screening phase the screening phase itself starts. However,
when a case's simulation has already ended during the pre-screening phase the
screening phase is skipped. Other than that, a case's simulation may also end
during the screening phase itself, ending the screening phase.

As long as the case simulation has not ended (i.e., the case's state is
tt(PRESENT)) a screening is performed for each of the screening rounds defined
by the tt(Screening) object (cf. section ref(SCREENINGCLASS)), initialized in
tt(Loop's) constructor.

At each screening round two actions are performend:
    itemization(
    it() given the round's screening age it is determined whether the case
        leaves the simulation at that particular screening round. If so then
        the case's state is no longer tt(PRESENT) and the screening phase
        ends for that case (see the next section for details);

    it() if the case is still present, then a screening is performed, which
        may also result in changing the case's state, which then also ends the
        screening phase for that case (cf. section ref(SCREEN)).
    )
