Oops, had to stop and try it out. Now I got an answer, and it makes me think I missed a slicker solution.
Find the coefficient of x^4 in the MacLaurin series of 1/(1-x)^3.
Why? What's the connection?
I'm going to guess that this is a mathematician joke; it works (you can see this if you look at the mathforum link someone gave above, plus the definition of a Maclaurin series), but the deep reason why is that 1 - 0 = 1. If 22B can tell us differently, though, e.g. give an interpretation for x that makes an interesting generalisation, I'm all ears!
