If anyone wants to try tackling this, a longer APL one-liner is referenced on the APL wikipedia page and discussed in length here:
http://catpad.net/michael/apl/ As an aside, APL was the first computer language I was exposed to. When I was around 7 years old my aunt (who lived in Boston near MIT, Harvard) had a computer scientist friend who gave her the "APL: A Programming Language" book, after she bragged to him about a smart nephew who liked typewriters... I liked all the symbols and sort of understood a few bits of it...
