In the chapter on functions a low of questions are asked in the style:
write a function such that the following tests work:

day_add("Monday", 4) ==  "Friday"
day_add("Tuesday", 0) == "Tuesday"
day_add("Tuesday", 14) == "Tuesday"
day_add("Sunday", 100) == "Tuesday"

However, these cause a lot of confusion because a lot of students in this stage of their education think these functions should return booleans.

Maybe we should rephrase these questions.