From the most recent update to the Agile2 PDF:

“I need to stress that the book is written on the assumption that the reader has Rails 1.2 installed. As that doesn’t yet exist, you HAVE TO HAVE EDGE RAILS for some of this stuff to work.” - Dave Thomas

This cart/sum thing is coming up a lot the last couple of days. The good thing is that instead of just copying lines out of a book, the tutorial makes you figure out alternate ways of doing things when they don’t work. The bad thing is there are a lot of people just starting out, getting really frustrated.