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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 might be faster--especially for larger numbers of handlers--since removing items from lists is O(n) on average vs. O(1) for sets, according to https://wiki.python.org/moin/TimeComplexity
Using sets appears to be a viable option since "you should not worry about the order of handlers' execution" and "each handler will be subscribed at most once at any given moment of time" as per description.