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Collections are a way for you to organize kata so that you can create your own training routines. Every collection you create is public and automatically sharable with other warriors. After you have added a few kata to a collection you and others can train on the kata contained within the collection.
Get started now by creating a new collection.
Ah, yes. I was about to make the same comment about ?*11=??. But you are right I missed this detail: "All of the ?s in an expression will represent the same digit (0-9), and it won't be one of the other given digits in the expression." I forgot to check whether the digit was the same as another one in the expression. Thanks!
Tests are perfectly fine. Read the description again.