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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.
Have you tried hitting the Submit button rather than Run Tests? My test case was just the function and gave me "Unknown Error." This is due to not using Test.assert_equals(circleArea(1), 3.14) function inside the test case window. But hitting submit uses the test cases of the author instead of your own, thus passing the test if your code works. Which it looks like it does.