Back in November I stumbled across this post about using
system:has:shortcut to find all one's bookmarks tagged system:shortcut:
http://tech.groups.yahoo.com/group/ydn-delicious/message/821

I had never seen this tag syntax before, nor could I find any
documentation on it. After a bit of fooling around, I realized that
the syntax works for any arbitrary tag system!

At first, I tried out adding moods to some of my bookmarks. For
example, mood:sleep-dep, mood:jubilant and mood:serendipitous. The
tag system:has:mood would retrieve all the bookmarks that had a mood
associated with them. This was neat, but not very useful.

Lately though, I have been using the "system tag" (or is it "tag
system?") syntax to identify projects. For instance:
project:homepage, project:shoppingCart, project:CSSrefactor. This
_is_ useful.

I can tag each bookmark according to the specific job it relates to;
but if I want to see every bookmark that relates to _any_ job, I just
look at the tag system:has:project. If want a quick list of every
project-related wiki, I can look at the conjunction
"system:has:project+wiki".

So now that I've discovered this very cool bit of del functionality; I
have two questions. First, is this intended functionality? Or am I
just enjoying a side effect that might disappear at some point in the
future? And second, is the system:has:* syntax ever going to be
supported in the delicious Firefox extension?

Thanks for taking the time to read my post, and thanks for delicious!

Noah