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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.
it's just a way to check the common elements between two sets: old groups the player plaied in and the new group. The only common element must be the player itself