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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.
In PHP, tests expect you to preserve keys of items which were kept. I would consider it a bug in tests, or at least an insufficient specification.