Yesterday afternoon, Flickr announced the release of a new feature that hard core Flickr users have been clamoring for for a while now: subsets or sets of sets. Or, as Flickr calls them, collections. Users can now build up large collections of pictures from building blocks of multiple sets, and even other collections. To differentiate them from regular sets when you're looking at them in the sidebar, collections have icons that are mosaics, with a number of pictures from the different sets that make up the collection.

As you can see from the screencap below, there are a couple of quirks. First, collections can only be nested 5 deep. for most people, this shouldn't be an issue, but some power users may hit that limit fairly quickly. Second, any collection can be composed of sets or other collections, but not both. That means if you have a collection and you want to create a new collection with that collection plus another set, you have to put the set into a collection first. There doesn't seem to be anything to stop you from creating a collection with a single set, so it's no big deal, but it is an extra bit of hassle, and it potentially costs you a layer of nested collections.

Finally, drag and drop editing of sets and mosaics doesn't seem to have the kinks worked out in Safari yet. When I clicked on the icons to arrange the mosaic on my test collection, the icons floated around the window nowhere near the mouse pointer. And the finished collection never showed up, even though everything looked fine. Firefox seems just fine, though.