/prog/ challenge #8489929

Create a list of /prog/ challenges. A /prog/ challenge should satisfy the following criteria:

1. It should be completable within 5 - 10 minutes by an EXPERT PROGRAMMER.

2. It should be completable within 5 - 10 days by a non-expert programmer.

3. A non expert programmer should become closer to an EXPERT PROGRAMMER after completing the challenge.

4. The challenge should stimulate interesting conversation, or perhaps elude to more interesting and fundamental problems.

First of all, challenge should be comprehensible to an EXPERT PROGRAMMER

Most challenges I've seen were badly defined. For example stating challenge as a simple "invert a list" may pose a lot of questions:
1. what a vague concept like "invert" could mean in the context of a list?
2. list of what?
3. is array a list or exclusively cons pairs list are meant?