As far as I'm aware, Delicious's plus operator doesn't simply add tag groups of URLs together, it produces the union of the tag identified groups. The union of any number of tag groups can be displayed but only the most popular of one. Hence, combining two popular tag RSS feeds from delicious wouldn't yield interesting results, because the distribution of URLs popular in group A and popular in group B rarely coincide, whereas it is these coincident URLs which I would like to have a feed for.
Are there any plans to add services which return the popular union of more than one tag?
