Is it an arithmetic progression?

You will write a function for the differences of a list, use it to test for an AP, and sort some lists.

The idea. A list is an AP when all the differences between neighbouring terms are equal. A list of one or two numbers always counts as an AP.

Your tasks.

1. In differences, return the list of differences terms[i + 1] - terms[i].
2. In is_ap, return True when all the differences are equal.
3. Set aps to the names in lists whose list is an AP, in the same order.