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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.
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In the description:
It's 'subtract', not 'substract' and it does work on codewars... you just have to spell it right.
For the first point, I strongly recommend you to read the documentation: https://docs.codewars.com/training/troubleshooting/
For the second, gmpy2 is available and you can import it without a problem.
Tests are fine, the problem comes from your code. Please don't raise an issue, an issue is a bug in the kata. You may ask a question but without knowing anything of what you're doing it's impossible to help you. I advice you to read carefully the documentation: https://docs.codewars.com/training/troubleshooting/