How can you derive the relationship between folders (or tag, though it's called "folder" in the viewer interface) and feeds through the Google Reader API?
There are ways to get the items that belong to a particular group/folder/tag/label, and you can check the item data to find out all the groups it belongs to, but… how can you tell which feed belongs to which folder? Considering that the viewer UI for Google Reader lets you sort feeds into folders, there should be a way to do this, shouldn’t there?
Is there a way to either get the folders that a feed belongs to, or to get a list of feeds that belong to a folder?