A good explanation was posted here a few days ago: http://news.ycombinator.com/item?id=1896643 http://news.ycombinator.com/item?id=1896643 (After looking it up

A good explanation was posted here a few days ago: http://news.ycombinator.com/item?id=1896643 http://news.ycombinator.com/item?id=1896643 (After looking it up, I discovered it was from the same site.)