Having the option to group cards in a deck. And also the ability to group decks of cards.

How about adding an option to group cards within the deck? And how about the ability of grouping different decks?

While studying a subject, you will find many sub topics within a topic. A sub-topic isn't big enough to make a deck solely for that topic. So if we can have an option to group these cards with related and relevant information within the same deck, I think it would be a boon.

Let's say we are learning new words. We can have a group of cards for a word and it's derivatives or we can have a group of card for all synonymous words.

Taking it a step further, how about having an option to group decks of related topics?

Taking it even further, hang on this is crazy, how about having the ability of a card to be part of multiple groups at the same time irrespective of which deck it belongs to?

Say I have a couple of decks for vocab. While studying I come across a word, and say I want to learn all the synonymous words, I can start studying the "synonyms" group that that particular card is a part of. Now say I want to learn the idioms with this word, I can study the "idioms" group to which that card belongs.

These concepts are in fact nothing but concepts of Set Theory!!