Okay I got tempted by the new Xorg v7 stuff, and despite the dire
warnings decided to "apt-get dist-upgrade" my sid box. Whilst I did
help discover a new bug with the install process I was left with a
system without X11, neither v6 or v7.

Pondered my options (I had done an "apt-show-versions" and saved the
output first, "just in case", so I know what was installed previously
including version numbers. So theoretically I have a recovery path,
but this path is not well trodden (for good reasons -- running sid
is for masochists)).

First I tried to backout changed to April 1st using;

$cat /etc/apt/preferences


Package: *


Pin: release a=sid


Pin-Priority: 1001





$cat /etc/apt/sources.list


deb http://snapshot.debian.net/archive/2006/04/01/debian/ unstable main

This means, for all packages, prefer the version in "sid", and setting
the priority greater than 999 means always use the Pinned version. The
use of such a high priority is typical of backing out changes, as apt
will then usually prefer the old version over the new. The exact contents of
sources.list will reflect what you had before you messed up!

This fails because later versions of packages from "sid" win out
over earlier versions of packages from sid. Thats the rules. I asked
around and drew a blank. In an inspired moment I did "s/sid/unstable/" in
/etc/apt/preferences (i.e. "Pin: release a=unstable"), and "apt-get update", "apt-get upgrade", and it
offered to downgrade 250 odd packages.

So as far as downgrading sid in apt is concerned release "sid" != "unstable", even if
we know better.

This is a terrible way to backout the Xorg v7 upgrade, reinstall Etch,
or wait for sid to be fixed. Downgrading is always unsupported and risky, as binary file format changes may eat your data. Don't do this at home^H^H^H^Hwork. Being able to downgrade sid to a specific days release is kind of handy to know, and similar things could be done in Sarge and Etch I'm sure.