Hierarchical ategories

The current edit deck function is useful for alternating cards, but less appropriate for sorting cards according to category. Additionally, categories cannot be more than one level deep. It would be useful to have subcategories so that, for instance, different languages that one is studying could be treated separately, but also different items within that language (eg, words for the next test, 50 most common verbs, writing system, etc.) could be sorted. Categories could be treated like directories, cards like files, and the edit deck could more closely mirror a file manager in its appearance, with a drag-and-drop interface for moving cards between categories. Perhaps it would be possible to switch between "sort" mode (the mode proposed in this blueprint) and "edit" mode (how it exists today).