Clay Shirky makes a very sound point in his interview with Adam Weinroth

The context of a tag is critical — users tag differently on del.icio.us than on Flickr, so treating tags as purely atomic elements strips them of much of their value. A ‘tag molecule’, so to speak, includes not just the tag and the URI it pointed to but the users, timestamp, service it was derived from, and other tags by that user pointing to the same URI. Getting that bit of encoding right seems to me to be the essential issue.

Web applications could exploit the full potential of folksonomies only if they can access all the information bits that, in Clay vision, should be packed into the tag molecules. Interactions between the molecules components will facilitate the emergence of patterns, relations, meanings. Clay seems to suggest that we need some formal agreement on a new encoding format, in order to start producing not just “tags”, but “tag molecules”. Maybe a microformat, something that will enrich the widely used RelTag.

But all the components of the tag molecule can already fit into RSS elements, without forcing the semantic of RSS 2.0 specifications It is not a new idea: Niall Kennedy proposed something similar months ago.

Each RSS 2.0 feed has an optional <category> sub-element with an optional domain attribute to designate a specific categorization taxonomy. You can have as many category elements as you need to with mixed or no domains. Using the category element with the domain attribute set to the best definition of the tag’s context allows for easy recognition of tags within the aggregator. It also allows a user to search their own database or create smart lists for all items with Flickr or Technorati tags, or build into the application a list of known tagging domains for special status.

For example, it would be nice if this very post could automatically generate an RSS item with the following structure:

<item>
    <title>Tag molecules? Build them in the RSS lab!</title>
    <link />http://www.clipperz.net/.../tag_molecules_build_them_in_the_rss_lab
    <description>...</description>
    <pubDate>Sun, 03 August 2005 15:21:36 GMT+1</pubdate>
    <category domain="http://www.clipperz.net/tags">tag molecules</category>
    <category domain="http://del.icio.us/tags">tag molecule</category>
    <category domain="http://www.technorati.com/tags">tag molecules</category>
    <category domain="http://www.clipperz.net/tags">original idea</category>
    <category domain="http://www.clipperz.net/tags">text only</category>
</item>

If tags could be wired in this way within RSS feeds, I’m sure this will generate a new breed of tag-aware RSS readers and tag-aware aggregators. They will sport new features to exploit the “tag molecule” concept.

The happy owner of a Ferrari “Enzo”, that is looking forward to buy the new F430 Spider, could configure his RSS reader to select only those posts about Ferrari cars, especially the Enzo and F430 models, but to ignore all the buzz about car racing, i.e. a selection of posts:

• tagged “ferrari” in at least one domain AND never tagged “f1” or “formula1” in any domain

• tagged “enzo” OR “f430” in the Autoblog tagspace

Then he can subscribe to the resulting feed.

Today aggregators could not offer such feature because they do not know the tag context. PubSub, Feedster and the likes just handle tags and not tag molecules. Of course a widespread production of tag molecules involves support from the blogging platforms. Their editing interfaces should give every blogger the chance to easily specify the couples tag/domain.

Drupal, for example, gives very limited options to handle tags. All the available modules convert tags into html chunks within the body of the post (hence within the <description> element of the RSS feed) and is up to the spiders of Technorati to recognize them as tags. No <category> is created.

Since Drupal is the software running Clipperz, I would be very happy to see someone in the Drupal community developing a new tag module with the capability to emit “tag modules” via the RSS feed. Maybe you autowitch?