Add the complete proof on proof cards to reduce friction when reviewing

i also now think that putting the full solution on the card, rather than just a reference to a textbook or web page, is better. it keeps everything self-contained, so when you're reviewing there is less work to do. in general, the idea to make reviewing fun.

When I started out with spaced proof review, I used to just add a reference to a textbook (e.g. textbook name and page number). This meant that when reviewing, I had to pull out the text and open to the correct page, etc., which was a lot of friction.