$ cat /etc/apt/sources.list
deb ftp://ftp.au.debian.org/debian etch main contrib non-free		# official etch
deb-src ftp://ftp.au.debian.org/debian etch main contrib non-free	# official etch

deb ftp://ftp.au.debian.org/debian sid main contrib non-free		# official sid

deb http://security.debian.org etch/updates main contrib non-free
deb http://www.debian-multimedia.org etch main

========
$ cat /etc/apt/preferences
Package: emacs-*
Pin: release o=Debian,a=unstable
Pin-Priority: 800

Package: *
Pin: release o=Debian,a=unstable
Pin-Priority: -1

=======
$ apt-cache policy libc6
libc6:
  Installed: 2.3.6-13
  Candidate: 2.3.6-15
  Version table:
     2.3.6-15 0
        500 ftp://ftp.au.debian.org etch/main Packages
         -1 ftp://ftp.au.debian.org sid/main Packages
 *** 2.3.6-13 0
        100 /var/lib/dpkg/status


=======
$ sudo apt-get install subversion                  
Reading package lists... Done
Building dependency tree... Done
Package subversion is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
E: Package subversion has no installation candidate