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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.
As explained above, to test for laziness
This is the whole point, the function should not produle all 1_000_000 numbers every time it's called. It should create only as many primes as it is asked for. If the function asks you only for numbers smaller than 10_000, and you still calculate prime numbers above 10_000, then it's an error in the solution.
The tests test for laziness.