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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.
Substracting inside an index is susceptible to "substract with overflow". The safer way to do this is with the saturating_sub() method but ideally you'd also want to check the length of the array (not required here).