Broke /etc/apt/preferences when trying to use just one package from unstable.

[James Youngman]

I modified /etc/apt/{preferences,sources.list} to get just
flashplugin-nonfree from unstable.      That seemed to work.
However, now I find that  "apt-get install libsdl1.2-dev" results in
this error:

# apt-get install libsdl1.2-dev
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package libsdl1.2-dev 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 libsdl1.2-dev has no installation candidate

Could somebody take a look at my preferences and sources.list files
and explain to me what I broke and how?    I attach both files
(/etc/apt/sources.list.d is empty).

Thanks,
James.

preferences
Description: Binary data

sources.list
Description: Binary data