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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.
better to do "type(x) is not str" # more readable
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You don't have to. The Kata says it takes a list of only strings and non-negative integers.