make-br-fn doesn't search for a literal _n, but it searches for anything matching the regexp /_(\d+|_)?/, except for anything of the form /_0+/. The free-variable checking code is based off of problems I did while working through Essentials of Programming Languages by Friedman, Wand, and Haynes, and it certainly shouldn't break in most common cases (especially since most common cases won't bind any extra variables). A second set of eyes is probably a good idea, though.
