This function evalutes a test function at run time to decide
whether to administer a series of test elements to the participant.
If the test returns
TRUE, then the test elements are shown,
otherwise these test elements are skipped.
Function to evaluate at run time.
This function must accept the argument
Either a single test element, a list of test elements, or a timeline,
which will be displayed conditionally on the basis of the outcome
A list of test elements, or equivalently a timeline, which can be combined with other test elements or timelines.