That is, the problem is to develop a function that a natural number and a list of symbols. Both belong to classes with complex data definitions, though, unlike for the previous two problems, the classes are distinct.
Figure recalls the two definitions.
Because the problem is non-standard, we should ensure that our examples cover all important cases. We usually accomplish this goal by picking one item per clause in the definition and choosing elements from basic forms of data on a random basis. The discussion on examples indicates that there are indeed four possible, independent cases that we must consider for the design of the function. Furthermore, the partitioning of the table yields four squares. Each square represents the case when both the condition on the horizontal and the one on the vertical are true.