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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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In the filter() you can use only the second condition as it covers the first one as well.
I know, but I find it more readable with parenthesis.
I like it. To be more minimalistic parentheses around bit shifts can be removed, because they have higher priority than bitwise AND anyway.
Certainty should be adjusted to a problem, there is no simple answer. For this Kata's task even 1 is enough as it should pass tests only once and won't be used anymore.
nice though will not work for case insensitive anagram (eg abba, [bAAB] )