Currently a card can only appear in one set of cards, but it would be useful to allow adding card to multiple sets of cards (for example clothes and sports or a chapter in the text book).

This would be similar like emails in Gmail can be organized under multiple tags/labels, and ideally those tags/labels could be hierarchical, like in gmail, so child tags could be created for storing the word("textbook/ch1", "sports", "verbs/irregular"), but when selecting sets of cards to learn, you could select either specific child tags ("textbook/ch1") or parent tags ("textbook", "verbs") to select broader group of words.