If r7 & library interface are widely adopted, some flavors of FFI could evolve within the library/srfi process and gradually become defacto standards..
Probably hoping too much.. but in any event..
Face it, by refusing to be opinionated they ensured Scheme will remain a toy.
Managing that, but still being flexible enough to be useful, is a huge amount of work, at least if you want to do more than just "call this c function".
