Notecards

I've given some consideration to several ideas along these lines before, and I'd like to continue that line of thinking in regard to Trello.

Set Theory

Cards are data points, decks are sets. Cards have multiple representations (full detail, card view, title view, custom views).

Columns would be sets. Really, mutually exclusive tags.

Hierarchy

Cards can have parent child relationships. This also automatically defines a 'children' and 'descendants' set.