MEP for implicit links groups

Hi all,

I wrote a MEP for implicit link groups using existing metadata:

http://projectmallard.org/mep/mep0020

Short version is that you can write links elements like this:

<links type="topic" groups="id:somepageid id:anotherpageid"/>

Or like this:

<links type="topic" groups="style:tutorial"/>

And it automatically assigns groups based on id or style attributes
without you having to duplicate that information in a group attribute
on a link element.

Fun side note: This is the first time I wrote a MEP using Ducktype
instead of XML:

https://github.com/projectmallard/projectmallard.org/blob/master/mep/mep0020.duck

It was unsurprisingly very pleasant to write.

Comments very welcome please.

--
Shaun