In the original code's prescreen handling the following condition is used:

    if (Nscr > 0 && (naturalDeathAge < scr[0].age || (tumor.isTumor()
         && tumor.selfDetectAge() < scr[0].age)))

Following this a lot of code is provided to be executed when the condition is
true. It's more elegant to put this code in a function, returning when the
condition isn't true. So:

    if (
        not (Nscr > 0 && (naturalDeathAge < scr[0].age || (tumor.isTumor()
         && tumor.selfDetectAge() < scr[0].age)))
    )
        return;

But this isn't too readabile. However, the condition can be rewritten using De
Morgan's rules. First the condition is simplified by using symbols:

    not (Nscr > 0 && (naturalDeathAge < scr[0].age || (tumor.isTumor()
--> not (    A   and (                B            or (      C

         && tumor.selfDetectAge() < scr[0].age)))
-->     and                       D           )))

And thus:

         not ( A and ( B or ( C and D ) ) )

Now distribute the 'not' using De Morgan:

         not A or not ( B or ( C and D ) )

The 2nd 'not' is next:

         not A or not B and not( C and D )

And maybe also the 3rd 'not':

         not A or not B and (not C or not D )

Now substitute the original expressions:

    // if (not A) ->
    if (Nscr == 0)      // (Nscr is never negative)
        return;

    // if (not B and (not C or not D )) ->
    if (
        naturalDeathAge >=  scr[0].age      // not B
        and
        (not tumor.isTumor() or tumor.selfDetectAge() >= scr[0].age)
    //   not       C         or                  not D
    )
        return;
