git-annex tries to ensure that the configured number of [[copies]] of your
data always exist, and leaves it up to you to use commands like `git annex
get` and `git annex drop` to move the content to the repositories you want
to contain it. But sometimes, it can be good to have more fine-grained
control over which repositories prefer to have which content. Configuring
this allows `git annex get --auto`, `git annex drop --auto`, etc to do
smarter things.

Currently, preferred content settings can only be edited using `git
annex vicfg`. Each repository can have its own settings, and other
repositories may also try to honor those settings. So there's no local
`.git/config` setting it.

The idea is that you write an expression that files are matched against.
If a file matches, it's preferred to have its content stored in the
repository. If it doesn't, it's preferred to drop its content from
the repository (if there are enough copies elsewhere).

The expressions are very similar to the file matching options documented
on the [[git-annex]] man page. At the command line, you can use those
options in commands like this:

	git annex get --include='*.mp3' --and -'(' --not --in=archive -')'

The equivilant preferred content expression looks like this:

	include=*.mp3 and (not in=archive)

So, just remove the dashes, basically.

Note that while --include and --exclude match files relative to the current
directory, preferred content expressions always match files relative to the
top of the git repository. Perhaps you put files into `out/` directories
when you're done with them. Then you could configure your laptop to prefer
to not retain those files, like this:

	exclude=*/out/*