Completed this lame-ass exercise. The difference between scatter and gather is still bemusing. But, figuring out the key is to use the example in the text of 12.4, which should give you a tuple, then send that as the 'single' argument to sum, should do the trick. Thankfully, I also found a place to cheat and actually collect the answers to this: http://en.wikibooks.org/wiki/Think_Python/Answers.