#f was distinguished from '(). User-level, cooperative
multi-threading was added. Source-level debugging became more useful,
and programmer's and user's manuals were begun. The module system
gained a high-level interface, which is still used today in more or
less the same form.
format, optional and keyword procedure arguments,
getopt-long, sorting, random numbers, and many other fixes and
enhancements. Guile also gained an interactive debugger, interactive
help, and better backtraces.