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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.
The instruction said [4,4, 3,3..] and that the order doesn't matter, your test check for [3,3,...,4,4] without sorting beforehand...
Given that 0 is a natural number, just saying that it's defined for them would be enough.
A "power set" of a set E is P(E), that is the set of all subsets (part) of E. Why is it called "power set" ? Because you can see it as the set of all functions from E to the boolean set (if True is associated to an element, it is in the subset) which would be noted B^E (if B is the boolean set) and thus has 2^(Card E) elements.
Of course the interpretation with lists/arrays is to handle those as sets, so you have to forget the ordering of the elements.