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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.
You do not need sieve for this kata, but the problem with your sieve is that you rebuild it every time for every test (also in a somewhat inefficient way), while it could just be built once and reused between tests.