+ Another good Scheme implementation is Chicken. For our purposes, this is in some
+respects superior to Racket, and in other respects inferior. <!--
+Racket doesn't have R7RS-small, and won't anytime soon. :-(
+Also Chicken's library collection seems stronger, or at least better organized and maintained.
+Other R7RS-friendly: [Gauche](http://practical-scheme.net/gauche), [Chibi](https://code.google.com/p/chibi-scheme).
+-->