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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.
I believe that modifying element in filter is wrong and should be considered as an antipattern.
Maybe few additional examples and tests would be good
Test for NaN is not valid (in the instructions we return false if we encounter "wrong" valu while test for NaN value expects NaN in result)
Additionally it only happens once per x tests as it is happening during random tests.