Probably a lot of them, if they tried. Because Ruby is so poorly specified, I frequently read the interpreter source code to figure out how things are supposed to behave. CRuby is really well-written in some ways and really poorly-written in others. It's poorly written in the sense that it's a painfully slow line-by-line interpreter. It's well written in the sense that the code is very clean and well-organized: I can usually find answers in the source faster than I can find answers in the pick-axe book.