A Del.icio.us Wish

In case you haven't noticed... I use del.icio.us quite a bit. I have a growing list of links, and I also maintain my blog rolls using del.icio.us link rolls. For those of you that have never used del.icio.us, it allows to you add web links, and mark them with tags. You can then show different lists of your links using single tags, or multiple tags. Really, del.icio.us is based around sets.

When you add a link to del.icio.us, you say what sets (called tags) that link belongs to. You can then list all your sets, or alternatively you can list the intersection of different sets to narrow down your links. Realizing that del.icio.us is really just a website for managing sets, I wonder why they don't have ability to display a tag's complement.

Using a basic numeric example, I might have two sets:

A = {0, 1, 2, 3, 4}
B = {3, 4, 5, 6, 7}

Currently, delicious allows me to execute the intersection of A n B (I'm using n instead of the intersection symbol which is an upside down U):

A n B = {3, 4}

This is great as it allows me to create link rolls like blog+Microsoft for my right hand sidebar. But what about the complement? Sometimes I want to see all items that have a specific tag, but don't have another tag. I can't do that right now.

B - A = {5, 6, 7}

That way I can look through all my links and see all developer links that aren't articles, or blogs. That would be cool.