There is! It's "U". I'm sorry, it is written in the tutorial text for the second level, but it should have been more clear. Thanks for playing!
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Ah! That makes sense, and makes the game a lot more fun. I missed the tutorial text on the first few levels, since it's quite subtle at the bottom, and when I noticed that text in later levels I assumed it was always just cool dice facts. Can't fault the game other than that minor communication thing tho; absolutely brilliant work.