Enumerating a context-free language

[mbrubeck]

http://lukepalmer.wordpress.com/2008/05/02/enumerating-a-context-free-language

[10ren]

aside: I like how he uses mul and add as production names.

I've always found the expr, factor, and term names a bit confusing (maybe because factor is the only one that's specific to its operator).