We have preferred content, which is advisory, and numcopies, which is
enforced (except by `git annex move`). What is missing is an expression
like preferred content, which is enforced. So, required content.

For example, I might want a repository that is required to contain
`*.jpeg`. This would make get --auto get it (it's implicitly part of the
preferred content), and would make drop refuse to drop it.

> I've implemented the basic required content. Currently only configurable
> via `vicfg`, because I don't think a lot of people are going to want to
> use it.
> 
> Note that I did not yet add the active verification discussed below.
> So if required content is set to `not inallgroup=backup`, or
> `not copies=10`, trying to drop a file will not go off and prove
> that there are 10 copies or that the file is in every repository in 
> the backup group. It will assume that the location log is accurate
> and go by that.
> 
> I think this is enough to cover Richard's case, at least.
> In his example, A B and C are in group anchor and have required
> content set to `include=*`, and D E F have it set to
> `not inallgroup=anchor`. --[[Joey]]
