By the way, I'm kind of surprised you didn't use 4-letter names for your stub section at the bottom. How can you suffer the inconsistency in such an otherwise consistent grammar? :-)
I console myself with the idea that they are terminals, and I can see if I forgot to define a nonterminal by checking that no examples contain four-letter words. Choosing a name for the sum nonterminal was much more painful :)
My friend and I keep a link to the following because of our similarly neurotic tendencies :-)