It's a terrible interview question, not least because I spent however long writing 4 pages about it and still don't know what it is. :-) But it's not made up:

It's a terrible interview question, not least because I spent however long writing 4 pages about it and still don't know what it is. :-) But it's not made up: this comes up in a real system and I would like to know more about the answer.