[johncowan]

Alas, no. There are quite a few other things that are needed in what is now being called the Foundations in order for adding features to be done through libraries. Scheme, like other Lisps, is almost all "library" and very little "language", once you get past variables, constants, and function/macro calls. The only "language" feature added so far (in R7RS-small) is that previously a vector constant had to be quoted, whereas now it is self-evaluating: you used to have to say (vector-ref '#(a b c) 0), whereas now you can leave out the quote mark.