[plt-scheme] v4.0 and immutable pairs

I've just posted a lengthy note on the PLT Scheme blog
(blog.plt-scheme.org) that explains again why we're removing `set-car!'
and `set-cdr!' in PLT Scheme v4.0. The post explicitly solicits
feedback that is based on experience with the new version.

For that, you need the new version. Our current plan is to merge the
current v3.99.0.2 with the SVN trunk on Tuesday (or so). Nightly builds
will become available as soon as we can make it work.

We're still quite far from a v4.0 release, and the SVN trunk will be
unstable for a while. Although most things work, lots of documentation
has yet to be ported to Scribble, Help Desk doesn't yet use the new
documentation, various DrScheme tools don't work, and many libraries
(e.g., in "mzlib") that should exist new places still exist only in
their old places. We're making progress, though, and shifting v4.0 work
to the SVN trunk seems necessary to keep things moving along.

To preview the v4.0 porting notes, see

 http://svn.plt-scheme.org/plt/branches/v4/doc/release-notes/mzscheme/MzScheme_4.txt


Matthew