I think this is the reference implementation for r7rs as well isn't it?   

On Thu, Dec 1, 2011 at 12:24 PM, Bakul Shah <bakul@bitblocks.com> wrote:
Alex Shinn's Chibi-scheme is a r7rs "small" language
compatible Scheme. It can be used standalone for scripting or
as a library to provide an extension language. Full guide at
http://synthcode.com/scheme/chibi/

Installation:
 hget http://chibi-scheme.googlecode.com/files/chibi-scheme-0.5.1.tgz | \
   gunzip | tar xv
 cd chibi-scheme-0.5.1
 mk install

Its plan9 support needs more work: access to more of plan9
API, mkfile needs work + targets for .a, html docs, more
tests, etc. but the port is eminently usable.