[mb-style] ARs for labels

[Lukáš Lalinský]

Hi,

I'm working on adding support for labels/catalog numbers. Now that I have
implemented adding ARs for labels, I started thinking about possible AR types we
will need. I can think only of these two:.

label--label:

 * parent label
   * <labelA> is a parent label of <labelB>
   * <labelB> is a sublabel of of <labelA>

label--url:

 * website
   * <label> has a website at <url>
   * <url> is a website for <label>

Any ideas for more?

-Lukáš