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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.
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The for loop starts from the first prime number (2) and if n would be divisible by non-prime i, it also means that it is divisible by i prime factors. So n would be reduced (n /= i) in earlier loop steps, n % i == 0 can be true only for prime i because of it.