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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.
Agree, definitely nice as it makes you think about the nature and sequence of primes (these abstract mathematical patterns can emerge themselves in the real-world quite often). Getting to know primes better is definitely a good exercise.
As for AntraxMiope, are you still stuck? If so, think about how to speed up your algorithm for prime checking.