Also, runtime behavior depends on the order of --add-module compilation flags! Got bitten by this when sub_filters weren't working... and then I realized that the order in which [un]gzip and sub_filters are applied isn't driven by the order of the directives in the config file, but by the compilation string - the substitution filtering was being done on gzipped gibberish!

This was probably a Good Idea (tm) for performance reasons - filtration is just a compile-time linked list of function calls. But it's barely documented at all!